Mario Kart Wii is an insanely fun game, but that's not to say it doesn't have any problems...
First off, the game is packaged with an accessory, the Wii Wheel. You insert your WiiMote into the wheel and drive like you would a real car. It works surprisingly well, and I would not go back to analog control after trying out this new feature.
Single player mode includes Grand Prix, Time Trials, VS, Battles, yadayadayada. Grand Prix is the highlight of single player, you race against 11 other computer controlled players to win first place! 50 cc is easy enough, but the difficulty is ramped up significantly in modes like 150 cc and Mirror Mode, which are absolutely brutal and frustrating at times.
Once you finish the Grand Prix, unlock some karts, bikes(a new addition) and characters, you'll be ready to hop online.
Online sets you up to race or battle against players from around the globe. One thing I must point out, Mario Kart Wii online has absolutely no lag! I have the crappiest internet connection in my neighbourhood, and this is an absolute success for Nintendo. Friend Codes are in effect once again, but racing with complete strangers is simple, and you should be able to connect to matches quite easily.
Offline multiplayer is available as well, supporting up to 4 players. But, the game tends to slow down when there's more players.
Overall, Mario Kart Wii is a great accessible game for both hardcore and casual gamers. It is an absolute must by if you own a Wii. Anyone can hop in and have fun playing.