Maybe it's just because I'm a Nintendo fanboy and not a Sega fanboy, but I found "Sonic & Sega All-Stars Racing" to be rather on the bland side. Oh, it was enjoyable enough for a little while, but it doesn't have the longevity and staying power of a Mario Kart game, nor is it as wildly fun or addictive. Strictly on its own merits, it's a reasonably solid game, though the items are boring and many of the courses are unremarkable (while the Super Monkey Ball courses are just plain annoying). Additionally, there's a little too much Sonic representation here. Yes, yes, I know the Sonic franchise is one of Sega's biggest money-makers (though heaven knows why), but was it really all that necessary to include Shadow, or Big the Cat?
Overall, it's a serviceable mascot racer, and probably better than most, but it doesn't come anywhere close to touching the glory of the Mario Kart series.
This one is really more for the kids, especially if you've already played Mario Kart. Its far less challenging and has a lot less going on. For the kids I think its a lot more playable as there is a much larger margin for error. If the older crowd is looking for a game to really get in to this unfortunately isn't it. Biggest flaw is that if you play as your Mii, other than in the vehicle yourself you're always pictured as a generic avatar and you can only see your standings by 1P or 2P which got a little annoying. The power ups are fun, vehicles as well, but give this one a try for the kids or some family fun time and not an experience in race gaming.