Halo for PC is an amazing game. The gameplay is good, the plot is good, the enemies are good, the environment is good, the weapons are good.
The PC version is not quite as good as the Xbox version, as the online multiplayer lounges are often empty, and the graphics have been dumbed down in order to allow the game to run on lower end computers.
All the same, this is a must have game for any fan of the FPS genre. The multiplayer is world class, you can fight people online (when the lounges are full) or fight your friends via a LAN connection (that is so much fun). If there's one game a I recommend purchasing its this one.