I mean, sure, it's got the physics, and it's got the platforming, but it doesn't use them together. The game seems like a stitched-together mix of Mario-esque platforming and straight on-rails barely-interactive running. Why can't I have something along the lines of hurling over a pit with momentum from a run? Of course, the boost doesn't help in this regard, because no matter how much "momentum" you program into a game, it all goes to waste if you can just mash a button and get back up to full speed.
Speaking of the level design, there are quite a few moments where I sat in one spot for probably ten minutes trying to figure out what to do. There are also a few parts I don't like, particularly any part with a lot of scripted running followed by a HOMING ATTACK OVER BOTOTMLESS PIT LOLOL segment that expects you to be fully alert. (Starlight Carnival Act 1 has this.) It might just be me, but if you set the game on auto-pilot like that, can't you at least get the player's attention before he has to do something like that? There was also one part in (was it Sweet Mountain?) where I was launched off of a booster ramp and was given the MASH 2 screen. However, I kept dying even though I pressed 2! It took me probably ten tries to realize that the game actually expected you to press the button multiple times, which hadn't been properly introduced before that.
Also, the cutscenes. Don't get me wrong, the humor works a lot of the time (even though I groaned at more than a few jokes) but after hearing Drummond and Griffith since I was five, something about Sonic's new voice doesn't feel right. It's probably just me, though.
However, I will say that the game has its fun moments, and it looks pretty to boot. The Wisps are also enjoyable for the most part. I just don't feel it was worth $50.