I've played Doom 3 but not HL2 yet (DAMN PAPER SLEEVES! HOLD MY CDS, DON'T SCRATCH THEM!); I got bored pretty quickly with Doom, because it was dark and linear. The gameplay was alright, but I found I wasn't really having fun; it was like a field trip through a nice engine.

That's just my opinion, though; I'm sure there are plenty who don't agree with me.