A winner in the X-box, and a strong competitor in the PC platform.
Personally, I enjoyed Halo for its depth of play, including opportunities for flanking and stealth, as well as the fore-mentioned real-world physics (vehicles too) which is an eye-opener. Also, I like the fact that you can't carry a lot of weapons in this game and there are lots of gear for you to choose from, including conventional military types and alien guns, forcing you to make strategic choices or conserve ammo to heighten the excitement.
One thing I really get a kick out of it is the ability to sneak up on those dwarfy little aliens and give knock-out kills, which is really satisfying and fun in a way. Anyone who's an FPS lover, don't leave this one out!
KingLim