Even though you can still have fun playing this game, there are plenty of questionable and disappointing decisions made.
+ This was the first real big transition for Sonic going from 2D to 3D. I'm sure it was hard for the developers to smooth everything out during the transition, which is understandable. The original Sonic games flourished in 2D space and had a very solid formula. When moving into a 3D world some of those components just don't transfer well.
The one unique thing that this game did was allow you to play through the story mode as six different characters (Sonic, Tails, Knuckles, Amy, Big, and E-102 Gamma). There is one overarching story in the game, but you can play through the perspective of each character to get a little different take on the events that happened. I liked how they took a risk and made a game where you play as six different characters, but at the same time I don't think it turned out exactly how they wanted. The good thing is that you get to experience a variety of gameplay styles with each character, but the bad thing is that you don't have enough time to grow and develop the gameplay ideas that each character possesses. And a crucial part of making a good game is one that progressively builds on the skills that you develop when playing, continuously challenging you and pushing you further. This never really happens in the game since you don't really get enough time to really appreciate each character.
The first character you play as is Sonic. He's fast, can roll into a ball, and has a special jumping attack which hones into the enemy. This honing attack is really fun to play around with because it locks on to nearby enemies and launches Sonic directly at them, also allowing you to chain these attacks while staying in the air. Sonic's levels involve speedy maneuvers and typically focus on one story-based objective at the end of the level.
The second character is Tails. While he does have some speed to him, his special ability is being able to hover up off the ground for a short period of time. This allows him to reach locations unique to him. He also has a powerful tail attack. Since Tails is Sonic's sidekick, his levels usually are the same as Sonic's except your objective is to beat the AI controlled Sonic to a destination point on the map.
The third character is Knuckles. Knuckles has a strong punch attack, but more importantly he can glide around in the air (and slowly lose altitude). He also can climb up on walls and dig in the ground in certain locations. Knuckles' levels are more of a search-and-find mission where you have to collect 3 emerald shards, which can be in plain sight, trapped inside an enemy, or buried underground.
Amy is one of the weaker characters. She only has one basic type of attack, which is to whip out a big hair brush or something. If she's running this can propel her high in the air, but you have to be running at the right speed. Her levels mostly involve running away from robots which are trying to capture her and making your way to a balloon to escape. She also has some mini games associated with her makeup brush (like a whack-a-mole type game).
Big is the worst character in the game and is a complete waste of time. All of his levels involve a fishing mini-game which is extremely awkward and tedious. It adds nothing to the game and his character could have been left out of the game and nothing would have been lost.
E-102 Gamma is a lot of fun though. He has a gun that can lock on to enemies and objects to destroy them. You just have to hold the attack button and sweep in a circle for it to lock on. The levels for Gamma are interesting, too, because they are all timed and the only way you can make it through the level is by destroy more enemies and objects (which award you with extra time). So it's a race and it utilizes your attack ability, which I thought was nicely handled. Unfortunately his campaign is extremely short, though.
Overall the game does have some flaws, though. The levels tend to play themselves so there isn't a whole lot of interaction needed. But there is a problem with running in the game. The walls don't really let you glide off of them and if you are sprinting along and then touch a wall at an angle it will stop you completely, which is a huge pain and completely drains all momentum in the fast levels. Also, the boss fights are insanely easy. They typically just take three hits and then they die, and some can be beaten within 10-15 seconds. Another problem is that in the free-roam part of the game there tends to be a very poor sense of direction. The objectives can be obscure and it's not always clear where you should be heading. There are little glowing orbs which you can find and see a hint of where you should go, but it seems like I was relying on them way more often than I should and it was always a hassle to try and find one. It would have been better to see clear objectives in the pause menu or something. Finally, each individual campaign is too short so you never really get to experience a complete game. It just feels like six mini-games instead of one big picture. I guess it was a design choice, but it was a little disappointing to me.
Story/Presentation: 2/5
+ The story is particularly bad. The plot itself is kind of weak and the only interesting thing about it is how it is told from six different view points, which is an original idea for a game story. There are some moments where the overlapping events don't seem to align with other characters, though, which can get confusing.
Still, the worst part of it all is the voice acting. It is extremely painful to listen to the dialogue. I cringed at every in-game cutscene when the characters were talking. The language was gaudy, the emotion of the voices were over-exaggerated, and there were way too many pauses and gaps between sentences. It's just a total mess and really brings everything down. Combined with the annoying pop music, it almost reaches the point where the game would benefit from being played on mute.
Graphics: 4.5/5
+ The graphics are still surprisingly really good for how old this game is. After 10 years I was still kind of impressed with how the main characters look. I can imagine that when it was originally released it was the absolute best graphics available and a technological achievement, and even today you can see some of the beauty. Obviously things have advanced beyond the Dreamcast age of visuals, but there is still a certain quality in the graphics that is very appealing.
Value: 4/5
+ If you play through all six characters' campaigns then the game can take you about 11 hours or so. The Director's Cut version adds extended missions and trials which can give you some extra gameplay time, but I didn't try any of those. On Steam the game goes for $9 which I think is reasonable. It might be better to be a few dollars cheaper, but it's still not an outrageous price to ask. And I do think that the game merits a purchase. It's not the best Sonic game, but there is a certain charm that the game has. It's simple at times, but part of the fun comes in that simplicity. It has some faults but there is still plenty to learn and appreciate from this game.