For the most part, this is the same game Xboxers are familiar with. The graphics have been upgraded for the PC, certain DirectX 9 features have been implemented, and it runs in higher resolutions than the console version, but all things considered, the improvements aren't that dramatic. While weapons effects and explosions are definitely more satisfying, most players seem to be turning graphics options down in order to improve performance.
The physics in Halo is very impressive, with vehicles and player bodies that respond very appropriately to explosions and bumps in the terrain. If you watch carefully, you'll notice that things like spent cartridges actually roll to a low point on the ground.
It still feels a bit too much like a console game for my tastes, in that menus and text look oversized for use on a TV screen - a small quibble I have with many games ported from console to PC.
The controls have been adapted nicely from the Xbox's dual analogue sticks to the standard FPS mouse and keyboard combination. PC gamers typically prefer the new controls, but it will take some getting used to for those who are accustomed to the Xbox gamepad.
Vehicle controls are somewhat different
I found the lack of options in the options menu quite frustrating. The trend is toward making MORE options available to players - not less. Granted, a lot can be accomplished if you know the appropriate command line parameters, but I thought that sort of inconvenience went out with DOS. It would sure be sweet if you could switch Pixel Shaders without having to add "-use11" to the Halo shortcut, or activate a screenshot key without the "-screenshot" command. PC gamers expect to have some real choices in these areas, even if the console version didn't need many options.
The PC version of Halo has been enhanced considerably in the multiplayer department. There are a full compliment of multiplayer modes including slayer (deathmatch), team slayer, king of the hill, and capture the flag, which seems to be the most popular type of play. Six new multiplayer maps have been added to the game and there are two additional multiplayer-only weapons - the flamethrower and the fuel rod gun.
Note that the split screen mode is no more, and many players are lamenting the removal of the cooperative multiplayer mode which they enjoyed in the console version.
Of course, vehicles are what set Halo apart from the bulk of shooters out there. Some of them can carry numerous players who take control of onboard weaponry and ride shotgun while the driver navigates. Entering a vehicle shifts you to a 3rd person perspective, but still grants you reliable crosshairs when you're in a weapons position. The air vehicles also add some variety and a lot of fun to the game, if not quite as well-implemented as the beloved warthog. I do wish it was possible to change positions in the vehicle without exiting and entering again.
Although it's not difficult to see what Xboxers have found so appealing about Halo, it does fall a little short of what the latest PC shooters are offering. You can carry only two weapons at a time, the prone position is absent, and the grenade system seems over-simplified. While some of the vehicles and weaponry are very cool, Halo doesn't boast anywhere near the range of equipment that Battlefield or several other FPS games have.
As of this moment, there is an unfortunate lack of tools to combat team-killing. The word is that the next patch will introduce standard banning and voting systems.
Perhaps the most serious blow is that Halo only supports 16 players maximum, while other games of this kind typically accommodate 32 or more. It undoubtedly reduces lag, but also takes some of the intensity out of the multiplayer experience.
I'm not a fan of single-player mode, but I should point out that Halo offers a very rich single-player game with unusually good AI and a terrific story line which will justify the cost of the game by itself for a lot of people. If you're like me and you go online as soon as you feel comfortable with the controls, I would recommend Battlefield '42 or something in the Unreal series over Halo. Despite some unique features, the multiplayer just isn't strong enough to get me excited. And then there is Halo 2, which could well be worth waiting for given that it's probably only months away.