I've had steam since it first came out with the Half-Life series. I very much like it, for the most part.
The biggest benefit for me, is my games are available to me, anytime, anywhere, any PC and in most cases Mac. I just need an internet connection to login to my account and I've got access to everything I've ever purchased. It auto-updates, without having to launch the game (seems E: D may be an exception to this, not sure yet) and is a single interface for mods, DLC, forums, friends. Saves your games to the cloud as a backup automatically (so you also have them anywhere you go as well). They also often bundle games together and offer good deals (bought the entire Civilization series for $50 at one point); have over 30 titles on it. They advertise well, many games I'd never hear of without it. Same as Elite; I never even heard of Elite until the Steam launch. Didn't even know it was going to exist - had only heard of that other 'SC' game that is coming out.
The only single negative I can think of, is as of right now, you cannot sell games you no longer want. So in my lifetime of using steam, I've missed out on maybe $500 in used games sales over 10+ years.... not worried about that.