I love steam!
There are constantly ridiculous sales on games (I've gotten great games at up to 90% off). There are sales basically everyday on SOMETHING, "weekend deals" - including being able to play some games free for a weekend, which is a great way to try before you buy. They also cool week or more sales events around twice/year - the christmas time holidays in particular.
The social elements are cool too. It's fun to see what my friends have been playing, when they are online (also has a chat program). I even use it in place of something like ventrilo for coop playing with a friend in SF even for games that aren't on steam. (Though steam has a way to link in non-steam games for social stuff and ease of game launch).
When I got a new PC last year, it was really easy to reinstall games on the new PC and only the ones I wanted. You control which you download and install, etc. and don't have to worry about discs and stuff. All your games are available "forever".
Oh, and it's a very easy way to get game demos.
The only down side is I own a silly number of games because I bought them so cheap. :)
I think I may be a fanboy.