Halo for the PC is cool in that the graphics are crisper. Unfortunately, the way they did things, you can't use antialiasing or any of the other nifty effects your spanking new video card has (well, technically it's feasible but requires changing .ini files and downgrading other graphics...TMI, I know), so say hello to jaggies. Graphics and models are not that improved from the XBox Version, so expect older technology in terms of modeling and so on. Still, it was very fun to play and the campaign/story more than made up for it.
They essentially bring the XBox experience to the PC...almost exactly, even! You can decide whether that's a good thing or a bad thing, but if you've played the XBox Version, you've played the PC Version.
Keyboard and mouse control, per a first-person shooter, is standard and much more exacting (to me, at least) than the clumsy console controls.
Now, the thing that irks me the most? They took out co-op play. What made it so fun on the XBox was the fact you could play through the whole game with a buddy. Despite the vastly superior capability of the PC to multiplay (with a full screen, no less), they decided 'that's not where they want to go'. This is not to say there isn't multiplayer...there is. From what I understand, it's pretty fun, too, but the couple of times I tried it, it was pretty abusive on-line. I'm no slouch in the FPS department, but it was pretty brutal.
So to conclude my ramblings: if you haven't played the XBox Version before, and it's going for $20, give it a shot. It didn't have much longevity for me; I played it through a few times, and have no compunction to reload that sucker...and I bought it for $40.
zen