This game is reason enough to buy a GameCube, and is without doubt the best party game available.
The main game is pretty addictive and sees you rolling a ball around a series of increasingly difficult mazes. Completing these levels opens up the party games, which is where SMB really excels.
Monkey Target is one of the most bizarrely addictive games I have ever had the fortune to play. You launch your monkey-in-a-ball down a ramp, open up the ball into some wings, and then hang glide onto a target, collecting bananas on the way. It's a turn-based game and is outstandingly addictive.
Monkey Race is one of the better 'karting' games available on the current generation of consoles, although there are only six tracks.
Monkey Fight is a manic one-button fighting game, and is great for non-gamers.
Monkey Bowling, Monkey Billiards and Monkey Golf are all good implementations of their respective sports.
This is one of the best value games I have ever bought, and if you have three friends to play with, this (and the necessary GameCube and four controllers) are without doubt the best investment you can make.