In my opinion, the finest Mario game ever.
The game starts out with Mario arriving at Princess Peach's castle to find that bowser has kidnapped her (again!!) and that the only way to save her is to collect power stars throughout the 15 levels in the game. And so off you go, jumping gamps, getting fired from cannons, finding penguins and collecting coins along the way.
The presentation is quite nice despite being an old game, and the variety in the levels keeps things fresh visually as well as in terms of gameplay.
The only real niggle is that the camera isn't all that clever in places and can obscour your view which can lead to a few annoying deaths. But that aside, Super Mario 64 is an excellent game that you need to buy.