You know when you get a game and it's so good you basically forget to eat and sleep because you play it constantly? That's what Quake4 was for me, i got it about a year ago (and played it on a pretty average system) and thought it was brilliant.
However, apparently it's crap for the XBOX360 (i assume you're getting it on a PC though) and i've not played the online modes but not heard great things about them, considering the Quake series have always been quite multiplayer orientated (i think) that's not that good, but the single player was awesome.
As far as i know, the project was headed by the same chap who made Doom3, which i also really enjoyed. This game is a very similar style but i thought the AI was better than Doom3, which, i don't know if you've played it, but i thought was pretty poor for such an amazing looking game. The enviroments/enemies are also similar to Doom3 and i prefered the Weapons on Quake4. It's not the longest game in the world though.
Anyway, i'm not a serious PC gamer so there'll be people out there who'll have far more refined opinions than me. Still, i've been playing games on different platforms since the NES and this was by no means dissapointing.
You could always download a demo, i'm sure there's one about.