In terms of in-game action, the multiplayer maps in Halo 2 provide an enjoyable experience. But if you're a veteran of games like Battlefield and Unreal Tournament, you're not likely to find anything here that you haven't seen before. The primary gain is just the chance to deal out some destruction and mayhem with Halo's unique flavor. And if the included selection of maps doesn't give you a warm fuzzy feeling, there's always the included map editor which should eventually result in the availability of far more (and far more diverse) multiplayer maps than you'll ever see playing on Xbox Live. On the other hand, if you were hoping to use the Games for Windows Live matchmaking services so that you could use your mouse and keyboard to meltdown some gamepad-wielding Xbox Live players…don't get excited. Halo 2 doesn't support the ability to pit PC players against Xbox players. For that you'll need to get yourself a copy of Shadowrun.
Ultimately, if you've had your fill of Halo 2 on the Xbox then there's not a lot to lure you to buy the game again for the PC. The visuals are improved over the Xbox version, but don't expect there to be a wow-factor associated with it. If it's looks you're after, there are far better options. On the other hand, if you passed on the Xbox and you're a fan of the PC version of the first Halo, then Halo 2 is well worth your time.