You know... I was a little disappointed with this game. After seeing that they revamped the physics from Episode 1, I was pretty excited about it, but after it playing it it doesn't really feel like they changed much.
Sonic 4 basically has no story, at least none of any importance, so I'm not going to bother commenting on that. The gameplay is slightly enhanced from episode 1 so that you can no longer stick to the side of walls and rolling into a ball has increased momentum gain. Still not as fast as the good ol' days though.
The level design... seems a little lacking at times. It feels like there are a lot more gimmicks in this game than there were in episode 1, especially with the new Tails-buddy-up combo thing. The music has improved slightly in my opinion, and they threw out the horrible boss theme from the first game. In it's place is a pretty a couple of pretty cool up-beat music tracks.
The graphics look really nice, although I'm not sure I prefer the 3D models over the 2D ones from the first episode. But getting back to gameplay, it's not bad, just not all I had hoped for. I give Sonic 4: Episode 2 a 7/10.