Sonic Riders is weak in a few places, but it definitely has a solid community fun factor in it's favor.

User Rating: 7 | Sonic Riders: Zero Gravity WII
While the controls are decidedly lacking on the Wii in that turns are frustrating and it is indeed easy to wind up screaming as you bounce off the walls, I'm really pretty surprised at the low rating GS gave this one. I purchase Wii games based on community fun factor; IE, will my family and I come back to it time after time? I find the graphics solid, the courses fun, and the variety of characters and gear worthwhile. Great soundtrack too. This is not a world class game, no, but it IS fun once you get the hang of it.
Yes, I agree with GS, the story mode IS lame, but so what? It's a racer, and that's what I bought it for (multiplayer competition is what these games are all about for me). I do not recommend this one for the younger set though, at least not on the Wii. The controls are frustrating to use, and when you first start playing, whew, it does take a LOT of effort to keep from throwing your controller across the room. Mario Kart Wii makes excellent and accurate use of the Wii controller, so SEGA can certainly stand to take note and do as well on their next release.

Maybe this game isn't worth buying (feels more like a $30 title than a $50 one), but it is well worth renting so that you can make that call on your own.