In comparison to the other Mario Kart games, this one's tops. In comparison to any other game - this game sucks.
The major problem is that fans of Nintendo are comparing Mario Kart Wii with other Mario Kart games. Graphically Mario Kart Wii looks great - as far as the Wii goes. Control-wise, I was unimpressed. Call me old fashioned but I like the controller based games, never have I liked steering wheel peripherals - thus partial bias on my part I suppose, for not liking the game.
The number of drivers you can select in the game, and types of vehicles are varied and there is quite a good selection, though I found myself using the standard kart more than any other vehicle because of it's moderate stats.
I cannot say a bad thing about the multiplayer, as it is fun. I did enjoy the excitement of the last seconds in a race, fighting for the finish.
The reason, I suppose, that I can't play this game for more than 20-30 minutes at a time is because of the repetition and horrible steering wheel. You can only turn a free floating Wii controller in the air for so long before it becomes, quite frankly, boring. If you must get your Mario Kart fix, go ahead, but bring a friend.