Surprizingly inventive for Sega. A little epic storyline and everything. It was good. I was surprized.
Story: Surprizingly. I *gasp* thought the story was okay, expecially sense the game seemed to merge with it so smoothly. You actually play as a robot named Emerl who is..unique to say the least because of his mysterious past and the way his personality changes depending on who he is with at the time. You see, Emerl is like one big metal spung, he absorbs and copys almost everything he is exposed to. Meaning, after Sonic discovers him and takes him to Tails, Emerl starts showing character traits of Sonic and Tails, so even reproducing there moves when they fight. Emerl is also powered by a chaos emerald, which of course, catches the attention of the one and only Dr. Eggman. During the course of the story, Emel trys to discover what exactly he is and what he stands for. Sonic and the rest are more side characters than anything else, but it's hardly noticable.
Gameplay: I liked this part the most. For a GBA game the action and timed fighting in a 3D arena went very smoothly. As a bonus you can mix up all of the plain characters moves in Emerl to suit your fighting style. There is also a multiplayer here for those who know someone else who has the game. There are 9 characters to absorb moves from Sonic, Tails, Knuckles, Amy, Cream&Cheese, Rouge, Shadow, Omega (one of the E-serise, I can't remember but I think it was Omega), and Chaos. They each have pretty unique move lists to boot (as an old time chaotix fan, I wish they stuck more characters in there, but it's just wishful thinking I know).
Graphics: Traditional style on screen, nothing very notable with the exeption of the 3D environments that are aren't too impressive, but good enough. Artwise they tryed something new with the box art. I liked it.
Sound: Worked well with the game. They even had a bit of voicework in there.
In conclusion, I'm happy with how it turned out. It's a good little fighter, Sonic or not. Made with a general age range in mind and it great to just mess with on roadtrips. Now if Sega can just keep this up they might gain more rep. back with future games.