This is a good game. It rocks in almost every department. It has a lot of speedy and high momentum levels, it has a lot of good platforming levels, good mix of exploration, good chaos emerald minigames. The game is actually a great classic sonic experience. The level designs are REALLY great, I absolutely love how fun the levels were designed with the exception of Cyber Station zone. The characters all feel fun and unique to play. Plus there's quite a lot of replayability with the multiplayer/battle mode, and hidden ending.
The thing that holds back this game from being great however, is the design from the bosses, particularly the end-game bosses. They drag on way too long, with a lot of waiting in order to reach the next part of a phase. A lot of them can be pretty hard as well, making me think they balanced a lot of the bosses assuming you'd be playing the game multiplayer.
Overall this is a great classic sonic game, save for the bosses, there's very little to not like about this game.