I got one last Christmas for the boy. It has been a great investment, its really a great system IMO. Of course, you dont have the eye popping graphics, but a lot of the games are just FUN. Get the WiiPlay cause it comes with a second controller. My Boy downloads classic NES games all the time with his Wii Points. The Cool thing too is it plays Gamecube games as well.
As far as the coolest games, I concur with everyone on MarioKart, it is amazing, and once you hook up the wifi, you can play online against anyone. The gameplay and connection speed, etc, are amazing. I havent had it "lag" once on me while im playing people from all over the U.S.
The boy also has SuperPaperMario which he likes a lot, i got the WiiZapper which comes bundled with a cool little target shooting Zelda game. I also got "Medal Of Honor 2" which, i feel is done pretty well to be a Wii Game.
The online stuff is neat too, you can participate in the online polls (something my kids are fascinated with), check the weather, read the news (its pretty cool, you can check out the news from anywhere, like Alaska, Japan, etc.) And of course, you can download games to the virtual console and play all of the old school games too. Plus, you can check out the "Nintendo" channel, and watch video previews of new releases.