Sonic and Sega All-Stars Racing is really !@#$% awesome. The biggest issue that I have with it is the lack of franchise representation in the tracks and songs available. The gameplay is great, the online is superb, and you already know about the character roster.
If you like Sonic/Sega and you like kart racing games, buy it as soon as you can. It's well worth it.
[16:59] <Knuckles> Mario Kart 64 > Crash Team Racing > Sonic and Sega All-Stars Racing = Crash Nitro Kart > Mario Kart Wii
Edit: I just had to quote this post because it is perfect and wonderful and more people should read this and realize how much Metal Sonic sucks in the general scheme of things (I didn't write it):
Metal Sonic is not a killer death robot because he hasn't actually killed anything canonically. This is my problem with his reception-- Metal Sonic is called badass, yet he hasn't done anything to significantly deserve the compliment.*
[Shadow], however, has ironically lived up to Metal Sonic's praise better than Metal Sonic himself-- He usurped Eggman without lifting a finger whereas Metal had to forcibly lock him up. [Shadow] was willing and could have allowed the world to be destroyed without doing anything whereas Metal descended into a childish fit over the revelation that he wasn't a real boy and raged ridiculously. [Shadow's] heroic efforts after the fact, from saving the world single-handedly to taking on the immortal incarnate of pure evil were much more admirable than Metal Sonic's oh-so awesome acts of kidnapping Amy Rose before ramming his face into a wall.
Puh-lease. That pissy dishwasher can go take a long walk for all I care. When it comes to hedgehoggian doppelgangers, Shadow cleans up on the basis that he has actually done something worthwhile in his career.
* This is, of course, all being leveled at his game counterpart. I have no qualms whatsoever with his brilliant performance in the OVA, even if he did seppuku himself. <3