I haven't bothered with HL2, or anything Steam for that matter. I have considered buying HL2 and then cracking it so you don't have to use Steam to play single player. The whole concept of being required to be online in order to play the game in single player bothers me. What happens if you lose your internet connection? Are you just SOL for playing the game until your connection comes back? Sounds too stupid to me.
Edit: And you might say, "Well, everyone has internet and if it goes down it won't be for long." I just went through hurricane Wilma and two weeks without internet. I played all the games I had purchased but hadn't played yet because I've been playing UT2004 almost exclusively for the past 1-1/2 years. Games like Far Cry, Max Payne 2, COD, Doom 3 and others. If I'd had HL2 I would have been SOL. Glad I didn't waste money on it.