If you have never played any of the halo games, this is the PC port of the first game. Ive gotta tell you, ive played all three halo games, and this is the best, hands down.
The storyline is great, brilliant sci fi stuff with loads of twists and turns, the gameplay is brilliant, theres great 3 way battles between enemies in the campaign, and theres loads of action. They made the master chief weaker in all of the sequals, but he's an absolute killing machine in this game.
But the very best thing about this game is the multiplayer. Unlike xbox live, its completely free, and you can join massive 16 player battles easily. The maps are great for both vehicles and shootouts, and the graphics are great.
I cant reccomend this game highly enough, by far in my opinion it is the funnest HALO game you can buy.