Overall I liked the game.
But it's different from Sonic Adventure in that there's no hub/city between levels. Also, you don't get to choose a specific character, you basically either choose the hero side (Sonic, Tails, Knuckles) or the villain side (Shadow, Eggman, Rouge). Once you pick a side you play as all three depending on the level.
Also, there are three different play styles:
-Sonic/Shadow stages: These are the normal Sonic stages, similar to how they were in Sonic Adventure. By far the most fun.
-Tails/Eggman stages: They both ride around in a mech shooting things. Remember playing as the robot in Sonic Adventure? It's like that. The stages are aight.
-Knuckles/Rouge stages: Treasure hunting. Similar to Knuckles stages in Sonic Adventure, but the levels seem bigger and it's harder to find the pieces of the emerald or whatever. These stages are the least fun IMO. They take too long and aren't that fun.
So you'll spend a lot of time being forced to play as Tails/Knuckes (if you pick the hero side) or Eggman/Rouge (on the dark side) and wanting to just get back to the Sonic/Shadow stages.
But that said, the game is fun and I'd recommend it.