MK64 is a fun game, simply put. However, if you have a GameCube, buy Double Dash instead.
Although best played with buddies, it was also fun in 1P mode. The courses were great fun, and the challenge level was just about right. I had my favorite Karts (Toad, Wario), I thought that the developers did a good job of balancing out each racer.
Multi-player was a scream, although in 4 player mode the screens were too compact for a TV less than 36 inches. Battle mode was surprisingly fun, given that there were only 4 tracks upon which to do battle.
I agree that Nintendo could have done better, but the game is certainly better than a 6.4