Like a roller coaster, it has it's highes and lows...
Sonic's game play can annoy you with a few glitches now and again, but when you start to notice what causes them, they can be easily avoided. I enjoyed the action stages at the jumping and hitting bumper sequences and if the running part of the stage was a bit more refined (like being able to move while jumping) it would have been great.
Shadow's game play is slightly different than what I expected it to be. For one I expected him to be faster and have guns like in Shadow the Hedgehog (I was on of the few who liked that game). Instead we get a jump kick combo and the often unused Chaos Drive abilities. His vehicles played much better than in Shadow the Hedgehog, however there were some parts in Dusty Desert that made you flip over and kill you; causing massive frustration.
Silver's game play was the most intuitive to me. He plays amazingly well and his puzzles and stages are well thought out. The kicker is that he doesn't move that fast which in turn lets you plan your jumps, look at your surroundings and take in the beauty of the stages. I just started playing his story and I'm loving it every stage. Kudos Sonic Team, you did a goodin'.
Now the town stages are very under developed. The map sucks and the slow down is quite annoying. Also, the people in the town are a little stupid when you try to talk to one and you talk to the other person right beside them. They could have been much better, the best part is the store in parts of the town where you can buy new abilities and techniques which is always nice in a game.
My overall dislikes of the game are as follows. Playing and Tails and Amy, The horrible twitchy camera (standard in all 3D Sonic games it seems), The constant and seemingly unnecessary loading screens and the absence of the option of a Japanese language track.
In summation, fans shouldn't be disappointed. The way I see it, Sonic games are more of an experience than just a straight forward game. The music, stages, cinemas and game play blend into a mixture that pours out Sonic the Hedgehog. If your a like me and bought that game the day it came out and you didn't like it, I'd suggest renting games before you buy them. As for me, it started slow, but when I spent more time with it, it was Sonic Adventure all over again.