Most played game on my DS!
First of all, it is pretty challenging, and getting gold cups and unlocking characters and karts are great motivation to spend hours with this game. The game gets markedly more difficult as you change CC.
If you get bored trying to beat a course, then you have 2 options:
1 - Go on a mission. The missions are FUN! There are many challenging little mini games to play, I thouroughly enjoyed them all.
2- Go wi-fi. The thing I love most about this game is the WFC. It's so much fun playing with people from around the globe and showing off your karts and characters.
So, in short, GET THIS GAME! It has so much to offer.