An acceptable port of the breakthrough FPS that was HALO: Combat Evolved.
Obviously you now have the opportunity to use a mouse and keyboard, which allows for greater flexibility and accuracy. I personally prefer a gamepad over the mouse/keyboard combo because it's more comfortable and I cramp up after 2-3 hours of straight Halo at my PC, but it doesn't really noticably affect the gameplay one way or the other. Well, you do miss out on the force feedback!
With the PC version you also have a HUGE jump in clarity and certain graphical effects like the lighting and textures are improved. Yet, beyond draw distance, some textures, lighting and the resolution the graphics are surprisingly similar to the xbox version. At the time of it's release however, it was perfectly acceptable and nice to look at.
Finally, the PC version does make some actual additions to the game. Most notably the online multiplayer, which is a blast and was missing from the original Halo. Added into the multiplayer is some great new maps (5 I believe but don't quote me on that) and two new weapons. You can also now fly banshees and use shades in the multiplayer. The banshees are fun, especially on some of the big new maps like "infinity" and "death island" but the shades are much too inaccurate for my liking.
The reason I didn't give halo a 10 for gameplay was because I really, strongly believe a 2 player online co-op campaign option should have been implemented. The PC's at the time of release were more than capable of handling it, the game had already sold millions and been rated top-notch across the board, I don't see what bungie had to lose by putting in the option so many of us system link xbox party goers would die for. The graphics I gave an 9 because even though they were improved, they weren't quite up to the standards some other games were setting at the time of Halo's release.
Unfortunately, I am doing this review quite late in the games life, like a year after it's release. This is due to the fact that I'm playing it a lot in anticipation of the xbox 360 as I sold my xbox and paid off an xbox 360 per-order. I also have a brand new HDTV that looks beautiful in PC games that support 1280x720. And that Halo PC does very nicely!