Zero Gravity: A one-up from the original
The "gravity" mechanic replaces the "air" mechanic. Which is good because you no longer RUN OUT of power and have to run along the track on foot. And those annoying fill up stations are gone too. Basically you never run out of "gas". You just gather rings and do tricks to power up your board and perform a Gravity Dive which is basically the "nitro boost" in this game. It's effective and is your best friend if you want to pull up to lead position in a race. The Gravity Drift is similar to the tight turn mechanic in Sonic Riders but more effective.
I admit that Gear Change feels over complicated, could have been simplified, or maybe the game didn't need it. On the plus side, using it strategically opens you to the many shortcuts available on each track. Keep in mind though that you do not actually need to use Gear Change to win a race.
The story in this game won't win any awards but this is Sonic, right? Just take it as you see it. It's better than Sonic 2k6's story anyway.
So to put it bluntly. This isn't a bad game. It's pretty high on the list as far as 3D Sonic games go. And is a competent racer. The Unlockables and multi-player should satisfy you for a while. But for any major skeptics, still rent the game. Don't knock it before you've tried it.