Got the game. Played the first 4 acts.
Not too bad, but definitely different enough from Colors and Generations to potentially frustrate gamers who have come to rely on those two games as their Sonic gameplay staple (like myself).
The 3d Stages don't play like Mario if that's what you're afraid of, but then again, they don't really play like sonic either. The 3d stage gameplay is like something altogether new. I don't hate it and really if it was skinned as a new platforming character it would definitely get praised for originality. Some of the level design really takes advantage of this new play style, but it's not consistent throughout (again, only played first 4 acts).
One of the acts I played was strictly 2d gameplay and for Mario haters it's bad news as its VERY much like a 2d Mario game. My wife never plays Sonic games (any of them, even Genesis ones) as she always complains that they go too fast for her. She grew up on Mario 3 and Super Mario World. Well one look at me playing the 2d act and the first thing she said was "Wow, imitation Mario" and sadly, I have to agree (2d acts only). The look, the pacing, the gameplay, the music, the ratio of environment to player size, all of it screams Mario. The impression really hits you hard as soon as you start the act.
It is all forgivable though and if this was the next game after Sonic 06 or even Unleashed, I'd be happy. But after seeing the potential of Colors and Generations, it's hard not to think of how much better the 2d implementation of those games were.
Still though, seeing the reviews it has gotten, they're being completely unfair. This game from the outset is MILES away from Sonic 06 and is already more bearable than Unleashed.
TL;DR
This is Sonic in another world that is basically not a Sonic Game, but it's not bad either.
I gave Colors and Generations 9/10. So far, I would give this 7/10, maybe even 8/10 if I'm comparing to platformers all-round instead of just Sonic games.
Spoiler Alert
Storyline wise, the game is totally Sonic Colors 2.