If you get past the dumb concept, Zero Gravity has some unique ideas going for it.
So, one of my friends bought this game, so I tried it, thinking it was gonna be like the first riders, a mess... but... holy crap, I was actually proven wrong.
First lets talk about the controls. You can use the wiimote or GC controller. The GC controller is a tad more precise, but the wiimote is fun to use for those who are looking for something 'different'. The controls are also customizable. So theres something for everyone.
Something important to note is to do the tutorial, if you jump right into the game, you'll get frustrated and turn the game off. Its not a pickup and play game. This was a drawback that turned many people off to the first riders.
So the basics, your riding on a board, and the gravity is your 'boost', replacing the inferior air gauge which was your fuel. That's already a BIG improvement over the first riders. You build gravity through tricks or containers. Its less tedious to earn than the first game, and the challenge is to use your gravity wisely.
Tricks have been nerfed to timed jumping, instead of controlling the trick with the analog stick. It sounds dumb at first, but ultimately works better. Still, I think they could've done something better for the trick system.
The gravity powers are something that I don't think get enough credit. One of the thing that I like are how they're use to get on different paths, allowing you to stick on to walls, and taking alternate paths. It's almost like combining a racer with the Mario Galaxy formula. Ricocheting off of rocks in 'uber cool gravity boost mode' is also a blast. Not everything is executed perfectly, but I give the game this, I don't recall any other racer that does these kind of things.
The game also offers the ability to upgrade your board during the race with rings. It's another neat concept, and the board are varied in designs quite well. They also allow you to take certain shortcuts, which sounds good in theory, but sometimes overpowers the importance of shortcuts. This part of the game might have been executed better with some design changes.
If I had any huge complaints with the game though, it would be the lack of features. The tracks (like last time) aren't in great volume. The single player is awfully short, and the wifi, while its there, is a ghost mode that will only be amusing for a few hours. The story is pretty bad, but its a racer so who really cares about story? If you aren't a picky gamer, these annoyances are minor.
But what it comes down to is this - the game is marketed at the casual audience. Sonic games, especially spin-offs like Zero Gravity, are suppose to be easy to pick up and play. This isn't the case, it can take a while to get use to the difficult controls. So when you invite some friends over, and ask them "should we play Mario Kart, or Sonic Riders?" what do you think they'll say?
So it doesn't make a good party game, but succeeds in something else... innovation. So at times it can be fun, but commercially, this game fails in what it tries to do, copy the success of Mario Kart.