For years now, I've bought the majority of my games used on EBay, and when I was finished playing them, I'd make a decision. If I enjoyed the game a lot and thought I might replay it someday, I'd keep it. Otherwise, I'd sell the game on EBay where I'd bought it, and recoup most if not all of the cost. In this way, I'd play most of my games through what amounted to free long-term rentals. Every once in a while, I'd make money. But recently, I've been buying games via Steam, and I've got to say - Steam has a lot of appeal. But Steam's download model eliminates the possibility of resale.
I got burned with that one not long ago when I bought Overlord via Steam, found out that it stunk, and couldn't unload the game on EBay. At least when I recently bought the steaming pile of a game that was King Kong, I was able to flip it on EBay.
So what is it about Steam that makes it so appealing to me? The games I buy are always there for me to play, and not only do they take up no physical space on my game shelf, they take up no space on my hard drive unless I download them, which I can do whenever I like after I've bought them. I just need to be more careful about buying lousy games.
Also, the recently-launched Steam Community has some decent features. And while they don't yet approach the features of XBox Live, they're slowly improving. You can instant-message other online Steam users through the Steam client, even while one or both of you are in-game, and the newer games (Portal, Team Fortress 2, Half-Life 2 Episode 2) have achievements. Personally, I just like people being able to look at my profile and see which Steam game I'm currently playing. Hopefully, Steam eventually provides some manner of web widget that users can embed in a website, similar to the one available to XBox 360 users. Parsing through my Steam profile for the info is doable, but honestly, it's too much work, and if they change things even slightly it will require me to re-code everything.
For the time being, I'll continue to get the majority of my games through EBay, but Steam is an increasingly tempting lure, and I'll always buy Valve games from Steam rather than over the counter.
On the other hand, I bought Half Life: Episode 1 at Target when it was new for less than it was on Steam. When I got the Orange Box, which includes Episode 1, I put my retail copy up on eBay. It didn't sell for much, but the buyer couldn't run it because I had activated it on Steam in order to play it, and Steam doesn't let you transfer your activation. It took me by surprise because I'd never bought something at retail, only to find later that I had no right to resell it down the road.
I guess it's a case of love it or leave it with Steam activation. And it's definitely a buyer (and seller) beware kind of deal....