I have yet to play any of the Half-Life games, but not a soul has ever spoken ill of them, especially Half-Life 2. But I still like Halo 3, only played some of the campaign mode, and the multiplayer mode once you get used to the different controls. And I cannot with hold my psychotic glee when I got my hands on a grav hammer - though I've been told the gravity gun near the end of Half-Life 2 is just plain fun too, launching bodies into other people. Yeah, sounds like my kind of weapon.
I, however, am poor and a 360 is not in my near future. Were I to get a system it would likely be the Wii due to the next wave of games being more to my liking (Fire Emblem, NiGHTS, and more).