It's fun enough, but lacking most everywhere else.
Story:
Think Sonic Adventure type story, with more robots. Sonic finds a device called a Gizoid lying on the beach, it follows his commands after he shows it "His power", so he takes it to Tails, who analyzes it and determines it can copy fighting moves and increases in ower by absorbing Chaos Emeralds. Sonic decides (Rather irrationally) that giving more power and ability to a fighting machine can only be a good thing, so he heads out around his world to collect emeralds from various friends and enemies, interacting with everyone from the moody echidna Knuckles to bat thief Rouge to rebuilt robot Gamma in a quest for emeralds. Along the way he gets into many arguments, all of which are solved by beating up the other party. Sonic's not the only hero, there are eight stories, each with one of the good/semi-good characters. In each story you play as the story's named character and Emerl. The story sequences are long and cannot be skipped, and can get tiring once you've already seen them.
Gameplay:
In Story you wander around a map and select white circles to activate story sequences, sometimes only one is available to advance and sometimes many are. In the actual battle your object is to attack you opponent(s) until their health drops to zero, which means they are KO'd. In survival mode each player has a number of lives, and if they lose all of them they're gone. In KO battles you get one KO point for every time you KO an opponent, and the first to reach the set number wins. In Timed battles you get one KO point every time you KO an opponent, and the player with the most after a set time wins. Attacks include basic punches, punch combos, heavy attacks that knock your opponents away, upper attacks that kick them into the sky, and special attacks that do a lot of damage but can be automatically blocked. The special attacks can get cheap very easily, if you just keep switching between one and another you could easily murder a single opponent, and more powerful ones could even butcher groups of foes with little effort. Against a human you'd have to be careful because they could do the same thing back, but the CPUs aren't that smart. They walk into your traps instead of dodging them, and don't really get smarter as the difficulty increases, they just get stronger and block more often. Your health is determined by a meter, and your opponents' health is unknown to you until they die, which happens when their health meter runs out. Health is drained when you take damage; different attacks do more or less damage. You also have a special attack meter called the ichikoro meter, which only seems to have one purpose: Once it's charged your next special attack will be super-charged and instantly kill anyone it touches, assuming they're not immune to it. Both health and ichikoro can be filled by healing, which is accomplished by holding the guard button. Most characters have rather poor heals, slowly filling health, ichikoro, or both, only a few are actually really good.
Good for who? The Gizoid. Named Emerl by Sonic, it can copy one move from every playable character in a battle it is in, whether you play as it, with it, or against it. Moves are stored in the form of cards; you can gain one card per move. Every character gives at least one card to every move, plus a few extras. Cards include those mentioned above, as well as style cards that have an unknown purpose, support cards that increase stats like power, defense, and speed, color cards that change how Emerl looks, and ultimate cards that are super-maxed out versions of normal moves. The ultimate cards are also terribly cheap, often capable of killing the enemy far too easily, but they're Emerl only.
All cards take a certain number of points, five for every star on the top of the card. Emerl can gain 1, 2, 3 or 5 points after a battle depending on if he wins without getting KO'd or even taking damage, plus randomly he might get an extra point. Those can reach up to 500 in total, and the more he has, the more cards he can use, and the more powerful the cards are that he can equip. Emerl can store up to three different move sets, and can hold up to nine of each card (Getting another one does not affect your card count at all), which can apparently be traded with other Sonic Battle owners. Emerl's moves can only be changed in Story Mode, and he can only gain cards in Story.
Besides Story Mode you can also play battle mode, which is where you can choose a character to play as, choose a battle type, choose a battlefield or let it be decided randomly, and choose your opponents, who are they, how powerful they are, and whose team they're on (If teams is on). Thanks to limited artificial intelligence the battles against CPUs aren't too hard, but if you play against a human the difficulty can and usually will increase. Humans don't wander into mines (Unless they're invisible), humans don't fall for slow moves quite so easily, and humans are less likely to waste time trying to block attacks. Not that blocks are useless, just less dangerous then attacking.
Battle Mode (And battles in general) have one thing about them that adds extreme pacing: Speed. Character have little enough HP that they get KO'd about once every ten seconds, which is fast even for a fighting game. And faster characters rule the day, Sonic (With high movement and attack speed, but low damage) and Amy (With high attack speed and great damage, but low movement) win the day over Gamma (With very high power, but no real speed in anything) or Chaos (Who moves painfully slow and attacks slow despite his power being the highest of all). Emerl also can use the best of everything and become ultimately powerful, even without an ultimate card to his name. It's terribly abusive.
There's a practice mode, where you can test out attacks on opponents who don't do much of anything and never die (Neither do you though), a Challenge Mode where you choose a character, then fight certain foes in 10-Ko battles to see how many points you gain from play style in order to gain a rank between C and S, a records mode that keeps track of wins/losses, accumulated Emerl move card percentage, and play time, an options menu where you can change a few game details and listen to the sound test (Annoyingly minus certain themes, like the Green Hill remix), and a Mini-Game mode where you can play games you've unlocked. Four are multiplayer games I've yet to try, and one is a version of Minesweeper starring Knuckles, which forgot that you should never be able to lose on the first move (When you can see nothing), as well as resetting your high score every time you leave and forcing you to turn off your GBA to return to the main game. Not good for the mini-game, but it was really just a pointless extra anyway.
Characters:
As said above, unbalanced. Ten total, including the fast characters Sonic, Shadow, and Amy, the flying characters Rouge, Tails, and Cream, and the powerful Knuckles, E-102r Chaos Gamma, and Chaos 0 (Gamma and Chaos are unlockable), as well as customizable Emerl. The speed characters are the most powerful since the can attack before anyone else has a chance to, the flight characters can dodge easily but are generally weak, and the powerful types move and attack slow, making them generally weaker unless you can control them well enough to land a few solid blows (Which is all they need to KO a foe). A bit more work on the characters could have made them better, but it's not game-breaking, they're still workable. There are also several story mode exclusive characters that can only be fought: Phi is a cloning character, who can copy almost anyone you can play as and fight as them, you never know who you'll end up fighting. Guard Robo is nearly identical in play style to Gamma, except his heal is slow instead of speed of light, he doesn't warn you when he's about to self-destruct, he's a dull grey instead of red and black, and there can be more of him at once. And the final boss (Dr. Eggman, no spoilers there) who can fly, self destruct, and unleash unlimited missiles and bombs, but isn't terribly powerful. None of those can be played as in any mode, and only Dr. Eggman is different enough to be really worth it.
Courses:
Eleven total, one more than characters. You start out with ten, one of which goes with every character (Sonic with Emerald Beach, Tails with Tails' Lab, Knuckles with Chao Ruins, etc.), and can unlock an eleventh (Green Hill) by beating the final story (Emerl's story) once. They're not terribly interesting, they come in three sizes (Small, big, and long) and feature a variety of ledges scattered around. There's a twelfth course that is (Once again) story mode only, and that's the Death Egg course, that looks really cool but is not normally playable, sadly.
Unlockables:
Chaos Gamma is unlocked near the beginning, Chaos 0 near the end, Green Hill at the end, and mini-games and new stories as you go along. Not really a lot in the end.
Strategy:
Not a lot, opponents will often fall for the same attacks and traps until they die, not too much challenge in the end. It's much more fun against non-predictable humans. There is a difficulty setting, but it doesn't increase CPU intelligence, just attack power and HP.
Difficulty:
Easy. As said before, you've got predictable CPUs. The hardest part of the game is likely Knuckles' story if you accidentally fight a Phi, resulting in playing as Knuckles against two terribly fast Phi's (Usually Sonic and Amy), but that can be avoided by resetting your game.
Technical Details:
The graphics of the battlefields are 3-D, while the menus, story map, and characters themselves are 2-D. 2-D characters in 3-D battlefields are terribly awkward, while you can attack upwards it doesn't always look that way. It needs a lot of work. The music is mostly okay, with only two notable tunes and only one really bad tune. Too bad the bad one is the main menu theme and the good ones are the hidden course themes (Green Hill and Death Egg).
Multiplayer:
Up to four players can fight each other as long as you have enough link cables and friends with Gameboy Advances. I have neither, unfortunately. I do know you can battle each other, play the multiplayer mini-games, and apparently trade Emerl move cards around.
Chaos Emeralds:
Chaos Emeralds are a major part of every almost Sonic game, and are typically unique from each other and from the rest of the gameplay. In Sonic Battle, emeralds are gained like in the Adventure games, you gain them as part of the story when you are given them by friends or take them from Eggman's robots. Each one gives you ten new skill points (Until you reach maximum) and increases Emerl's intelligence, but only so far as the story goes (He won't have five emeralds of intelligence if you return to Sonic's story, where he had one emerald).
Replay Value:
The best part of the game is multiplayer battles, after that the best part is actually the story mode. When that's over you can replay it (If you can stand seeing the same cutscenes over and over) or battle a lot, but it does get old.
Final score breakdown:
Story: 8/10
Gameplay: 8/10
Graphics: 6/10
Sounds and Music: 7/10
Fun: 9/10
Replay Value: 7/10
Speed: 9/10
Pros
Battles are fast, if you like that.
Story is really interesting and worth seeing at least once.
Playing around with Emerl's abilities is fun.
It's fun beating up your Sonic friends.
Sonic Team wasn't evil enough to make Big the Cat a playable character.
Cons
Battles are too fast, if you think that's bad.
Characters are unbalanced.
Emerl is easily made over powerful.
The unlockables are dismal.
The music is uninspired, though not horrid.
2-D graphics in a 3-D battle don't work.
Overall: 7.4/10
The gameplay itself eventually wears thin, but it's quite fun for a while. The big complaint is in all the extra features, there's a lot Sonic Team could have tapped into to let you have more characters then 10 and more stages then 11. Even adding just a playable Dr. Eggman on his Death Egg stage would have been something worth talking about.
As it is, it's a Sonic fighting game designed more like a Sonic adventure game with fights instead of levels. It's still fun for a while, but could have been better.