My boys (9 and 7) have GameCube and a lot of games, basically all the Mario games. Mario Superstar Baseball is a lot of fun, but far less sophisticated than all the realistic MLB games out there for PS2, and XBox etc. But we play Mario Baseball all the time. Mario golf, tennis and soccer (super strikers) are all fun too. I actually picked up "Namco Museum 50th Anniversary" for GameCube which contains exact versions of about a dozen early-80s arcade games (PacMan, Pole Position, Galaga, Galaxian, Dig Dug, etc.) It comes complete with a title screen that plays random 80s tunes (Loverboy, the Romantics, Fine Young Cannibals)
If you picked up a GameCube cheap, good for you... Don't let anyone else tell you it sucks or you wasted your money... just have fun.
Personally I'm a wrestling die-hard so I can't play any wrestling game other than a Nintendo-based game. I just love the playability of the engine that Nintendo wrestling games have, ex. No Mercy, Wrestlemania, etc.