My favorite Mario Kart game of all time was Mario Kart 64. I liked Double Dash but I never thought anything could beat the great Mario Kart 64...Until now. With 32 tracks, 24 vehicles, 18 characters and a Wii Wheel included with the game, Mario Kart Wii is well worth $50.
Bikes
The gameplay in Mario Kart Wii has changed quite a bit since the past titles. I'll start with the addition of bikes. Now you can choose from having a normal kart or the new bikes. Bikes handle and drift harder, while karts go faster. Bikes can perform wheelies on straightaways to gain speed, but can only power up to a stage 1 turbo. Karts can't wheelie but they can power up to a stage 2 turbo. Also, to perform a mini turbo in a drift, you just have to hold a drift for a certain amount of time. This is to help beginners understand the game better.
Tricks
Another addition to Mario Kart Wii is the trick system. As you jump off of a ramp, just flick the Wii remote and the character will perform an awesome trick. How are you awarded by this? Upon landing, you'll get a mini turbo similar to the one you get by drifting. This is a good new addition, but a more elaborate system would've been nice.
Tracks
The game also follows Mario Kart DS's example with offering 16 new tracks and 16 old tracks with it. The new tracks are just amazing, utilizing every new feature the game has to offer. The old tracks, on the other hand, are not great. You can see where the pixels were in the SNES and 64 courses, and the GBA courses suffer from many framerate issues. They just feel more polished than redone completly. Still, with 32 tracks you can't complain.
The Racing
Even the races themselves have changed quite a bit. Now, instead of 8 racers, you have 12. This changes the nature of the game completely, and calls for different point values assigned to each position (1st place gets 15 points instead of 10). There are also new power-ups, too.
Graphics/sound
The graphics are, for the most part, really good. The old courses leave something to be desired, though, and the detail could've been improved a little bit. It still looks really good, though. The music has a good quality to it, although some of the music is repeated throughout some courses.
Control
Mario Kart Wii has followed Brawl's example and has offered 4 controller configurations (Wii Remote, Wii Remote and Nunchuk, Gamecube controller or Classic Controller). The Wii Remote feels similar to Exite Truck, while the Wii Remote with Nunchuk will fell strangely similar to Mario Kart DS. The Gamecube Controller doesn't work very well, though.
Wii Wheel
I just rented the game, so I didn't get the Wii Wheel, I'll update you on it, though.
Mario Kart Wii is great. It offers the best of all the old games and then some. You'd do good to buy it. I know I'm buying it.