This is a classic N64 game. The original Super Nintendo Super Mario Kart was one of the best things to make it to the 16-bit world, and MK64 improved upon that original concept a thousand fold. The graphics are sharp and clear, the courses and weapons are innovative, and it's just plain fun. You really feel like you're racing down the freeway in Toad's Turnpike, and the psychadellic colors in Rainbow Road (my personal fave) will really freak you out. Vroom, vroom, baby!
The one player game is pretty fun on its own, but my favorite option is 2-player. That way you can both race in the Mario Grande Prix for trophies. The 3 and 4-player modes are OK, but the screens are a little too tiny and you can only access the VS. and BATTLE options. Why can't 4 players race in the GP? That would've been so cool!
Sound is also well done. All of the characters have distinct victory shouts and cries of pain. Bowser roars, Mario says "Yippee!" and Princess Peach says "Yeah! Peach has got it!" They're all nice effects, with the exception of Toad, who kind of reminds me of the whining baby penguins in Sherbet Land. You'll probably feel like shooting him on occassion.
MK64 is one of the best games you can get for the N64. If you don't have it, get it. This is one race you'll want front row tickets to.