I love Steam. I've never had a problem with it. The ability to pre-install games before they even come out, while arguably pointless, is still a nice feature when it lets you play new games instantly. I've also gotten some good gameplay out of achievements (nearly vital for Left 4 Dead). Probably the most useful feature though is the ability to just log in to a computer and install whatever game you want, without worrying about anything. I recently accidentally broke my install CD to Jedi Academy, and now I'm thinking about getting it on Steam for $10.
In fact, I don't really remember the last PC game I bought that wasn't on Steam; unless you count the Indie Bundle... which I wish was on Steam for the achievements.
The only consistent complaint I ever hear about Steam is that they have a monopoly on digital distribution methods, and whether that's true or not you won't hear me complaining about it.