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