Half Life 2 is one of, if not the best Xbox game on the market.
The story line is gripping and makes you want to keep playing, but in the end this just makes the cliffhanger ending all the more painful. (luckily HL2:Episode 1 and HL2:Episode 2 are out now, so if you have a competant PC you can get those.)
HL2 also has quite a good set of weapons. Here are the weapons, in order of appearance: crowbar, pistol, SMG, grenades, magnum, gravity gun, shotgun, combine rifle (machine gun), rocket launcher, crossbow (sniper rifle), pheropods (to command the giant bugs.) The gravity gun is extremely fun to use. It has been crowned best video game weapon ever by some gaming website, I cant remember which.
Half Life 2 isn't all good though. At one point in the game you have a squad of up to four guys that will follow you and help you. You can give them limited 'go there' and 'follow me' commands but that is it. The ai can be dumb and your guys will get caught on door frames sometimes and they will run full tilt into mines without a second thought. This is only one portion though, and doesn't bring the game to its knees in the least. While Half-Life 2 on the Xbox is a great game and a smart buy, an even smarter buy would be to wait for Half Life 2: The Orange Box to come out. Only if you have a PS3, Xbox 360, or good PC. HL2: tOB includes Half Life 2, HL2: Episode 1, HL2: Episode 2, HL2: Portals, and TeamFortress 2.