This is already a great game, so I will comment on how it is on PC. It is a great multiplayer experience; those familiar with the Xbox360 Live system will feel right at home, especially since this pc version features the achievement system from the xbox360, and the Games For Windows Live system, which Halo2PC uses for online play, is nearly identical to Xbox Live. You can even import a Xbox Live account to use for Halo2PC. The only real problem with this game is that it sometimes runs a little slow, particularly in campaign, even on a high-end system. This is because this game was ported, and thus was not optimized fully for the PC. Had it been, it would have been smooth as butter 100% instead of 85%-ish. The game comes out of the box with Xbox 360 controller compatiblity, and I would not recommend playing without one, not because the game's non-controller controls are bad, but because using the controller is so much better.
Also, in-game voice-chat is good, in fact identical to the Xbox 360's voice chat system. Unfortunately, no spilit-screen action, co-op, or interfplatform play with the Xbox version.
That said, if you have the specs, don't hesitate to buy this game. There are still many who play it online.