I have had steam for many years and there are so many advantages to it. I own over 250 games now, mostly bought at less than 5 dollars on their 2x annual sales.
One of the key things I like is the match making ability in games that support it. All I do is hit shift-tab and invite my friend. It is so painless compared to many other games. Everyone on steam has the same version, no compatibility issues. Never had a problem playing multi-player games.
Mods are fantastic. Look at Skyrim, all you have to do is browse, and subscribe. It could not get any more easier. I hate going to different sites like Civ fanatics etc... It is so much nicer with steam. I have not bought a game in years retail. Picked up some great games for cheap on the July sale last week. I think I paid $15 for the Witcher 2, that was the most expensive game, the rest were $5 or less. All the expansions for Fallout Vegas were 75% off. If you don't know all games go on sale in July, and at Christmas time. People that don't like Steam must have money falling out of their pockets, or just don't play enough games to justify it anyhow. I however am completely in love with Steam
I wish something like this existed in the 90's.....
What is there not to like about it? I have NEVER had a problem with steam, and never had a problem playing games offline (unless of course its multiplayer). I often buy two copies of games so I can play with my son, they are such a good deal.
I also use Gamefly (Direct2Drive), Gamestop (used to be Impulse), and a few times Amazon.com. I always regret buying games from any other service, I really don't like them.
I also have repurchased several games just to have on Steam. Most recently I purchased Borderlands, think it was 5 bucks (or maybe 7.50) with all the expansions .... why? Because when my hard drive crashed, I had a hell of a time getting my DLC re-activated again, it took weeks! I had to email them so many times, and it was almost impossible to find the place to re-download it. What a nightmare! It had to be one of the most frustrating experiences ever. On steam, its simple. Just right click on the title, and it knows what DLC you own.