Seems better than both Unleashed and Sonic Colors.
The modern mode still has side scrolling. It seems like you can't just hold turbo all the time - not because you fall into a hole, but because you blow past other paths that presumably have more rings and extra lives and stuff. It feels pretty similar to Unleashed in those sections, but smoothed over.
The Classic level actually feels like the old games, unlike Sonic 4. I think that the jump rises up a little too sharply, and the camera follows it too eagerly, but I was used to it by the end of the demo. No physics glitches, lots of branching paths, enemies with smart placement, etc. Felt like the old games. I am pretty sure that if you hold down to roll you lose momentum though, and you come out of the roll earlier than you did in the old games. Not a huge deal to me.