This game definitely isn't my favorite Sonic, but it's okay. Playing as Sonic feels like a hold-forward adventure sometimes and feels like there's just one route you're supposed to go, and then jumps to amazingly-broken once you get the Sky Gem or Purple Gem (hax jumping/infinite jumping, respectively). Still though, the stages have nice design to them, and I do like the mach stages and their out-of-control feeling.
Shadow's part of the game is okay, I suppose. I haven't played around with it very much, but I can stand it, unlike ShTH. :(
Silver surprised me actually. He's the character that controls the best out of all three, mainly due to his speed being the slowest. This character is probably the reason that Sega used the Havox physics engine in this game. Also, his hover technique acts like Blaze's does in Sonic Rush. His voice may be somewhat young and annoying at times, but he's definitely one of the more fun parts of the game.
There are parts that I hate and just ignore the existence of, but the parts that I like are fun and provide a good enough challenge and reason for me to play the game. This game is in between Adventure 2 and Heroes for me.