I love Steam, a lot. It's the only DRM that I don't hate, because it offers me real incentives. I hate DRM, but with steam, it's as if it isn't there. Offline mode works fine, sure, you need to be online to enable it, but once you are in offline mode, you could keep it that way for the rest of eternity. The every day deals are awesome, the holiday special was amazing (I purchased around 50 games during that time period.) The only time steam has ever really annoyed me, is that if my connection is very slow, launching steam is a total pain. It takes almost a half hour to get things running smoothly, i don't know if this is normal for everyone with slow connections, or just the nature of mine (A very painfully piss-poor wireless connection.) However, I'm not going to fault steam too much for it, because it was probably more the connections fault.
I have around 150 steam games, and while some of them may not be the greatest games, i regret none of my purchases, as the ones that aren't great I got for so cheap, I can't complain. Plus, Steam community INGAME. I love it! I've experimented with one program which would allow me to use a web broweser inside a game, didn't work for shit. I love that feature so damn much. Plus, the clock is great too, I tend to lose track of time while I'm playing, and now I don't have to alt-tab out to check. (Also helps with non alt-tab friendly games.)
The social networking features are pretty standard, but good. I wish there was an integrated Meebo/pidgin omni-network chat client. I can use meebo in the web browser, but there is no tabbed browsing, so it gets to be a pain to have a wiki up and meebo up at the same time.
Still, what it offers is great.
Seriously, I fucking love Steam.
It also usually doesn't interfere with the mod-ability of a game, which is nice.
That said, Steam has learned a lot from it's primary competitor, Impulse, and while I do not USE Impulse, as far as the need for competition goes, I hope that Impulse sticks around for as long as Steam does. I don't think Valve would turn into an evil company if it gained a monopoly, but hey, let's not find out
TL;DR:
Valve does good things very well. Steam doesn't disappoint.