Racing at its best. 'Nuff said.
Mario Kart Wii improves on the bullet-proof Mario racing formula in nearly every conceivable way. There are more characters and karts than ever before (not to mention the addition of snazy new motorcycles), and the level design is nothing short of marvelous.
Above all, MKWII's most valuable asset is it's motion control driving. Virtual steering adds a wildly fun new element to Mario Kart, and the controls have never been easier. The included driving wheel attachment for the Wii remote makes $100+ driving wheels for other systems seem ridiculous and bulky.
I'll admit, the wacky cartoon feel of Mario Kart may not be fore everyone, (especially if you're prone to motion sickness) but I find it much more interesting than Mario Kart's realistic racing competitors.
If for some reason you have a Wii, but don't have this game, I suggest you cleanse your soul in Valhalla, and then visit your nearest video game retailer. You don't know what your missing!!