I played a little bit of Half-Life 2 for the Xbox. It is getting good marks, which, having finished the PC version, is understandable. While the overall experience of HL2 wasn’t was good as HL1 (HL1 is an amazing game), HL2 was really good.
All that said… if you don’t have a PC or a PC that is up to snuff to play HL2, the Xbox version is probably a reasonable compromise, but, if your PC can handle it you should definitely play it on the PC. The PC control (mouse + keyboard) is much better, the graphics are much better on the PC, and while the PC loads the levels in a streaming fashion (i.e., no “Loading” screens) the Xbox version displays loading screens constantly. The one last feature that seems to be woefully missing (maybe I just overlooked it?) is “Quick Save” — on the PC you can just hit F5 (or some such) key at any time and you are back playing in a second or so. On the Xbox you have to press several buttons, it is really annoying.
Anyway, if you like FPS games you need to play HL2. I have seen the PC version going for really cheap lately, but, keep in mind (with the PC version), if your hardware isn’t up to snuff the $30 – $50 you will spend in the game is just the tip of the iceberg.