Solid on the Xbox, but could it be better on the PC?
First off the graphics, running on an NVIDIA 6600 GT, look great and the textures don't pop as they do on my Xbox. Go ahead and crank up the textures, particles, specular maps, etc in the video settings and enjoy.
The next improvement I find when I play on the PC is the ability to kill the game's music and play another music source. By far, my favorite music to listen to while playing Halo on the PC is Groove Salad. You can stream it for free from SomaFm.com. I use Winamp. It's a cool mix of music that really goes along with Halo gameplay. For me, it really has the vibe of Halo. Try it, you just might like it.
Some might find it easier to target enemies and pull off headshots better with the mouse. I play it so much on the Xbox, I really don't notice.
Mulitplayer has some added features, such as the flamethrower and the Warthog with the mounted rocket launcher, instead of the machine gun. I find the flamethrower to be useless and can understand why it was removed from the Xbox version. You can also play over the internet through GameSpy or just hookup a bunch of PC's for a LAN party (the latter is a little harder to coordinate).
There are some things that the PC version can't give me that my Xbox version does. They include the ability to kick back on my couch, play the game in surround sound with 125 watts per channel, and use my 36" Sony TV. On my PC, I am limited to sitting in my office chair in front of my 24" monitor and listening to the somewhat acceptable audio coming through the Altec Lansing stereo speakers.
All in all, I find the PC version of Halo to be another great game to own. Right now you can probably pick it up for around 20 bucks brand new from the store.