Short answer? Good god it's awful.
Long answer? Glitchy, horrid hub world, speed stages are abysmal, Silver sucks, there's this weird kinda romance between Sonic and a human girl... Yeah... No. Don't play it. Only redeeming quality is Eggman is actually kinda evil.
If you want a good 3d Sonic game, pick up Generations, or download the Adventure games off of PSN or Xbox Live.