Sonic Riders was a great idea, but, when it went from idea to game, I think something got lost in translation here...
Graphics: The game really shines here. Graphics are second only to Halo 2. Sega really spent time with this department.
Sound: A great soundtrack, a cool, if not annoying, commentator make this part all the more sweeter. However the voice acting is sub-par.
Overall, the game is a great rental, but only if you know you will love it should you consider it for purchase. (And if you have the OPPOSITE of a short temper...)