Yes, I'm using Steam (for Left 4 Dead 2 plays with friends) and yes, I'm enjoying with it. Why?
Because it's easy to use, often there are incredible free-games (like Portal, for example), but I have friends, no worry for patch. Steam simplify my life as videogamer, so reading this article on Gamasutra make me think (a lot).
Steam is a DRM system and DRM (Digital Right Management) from user point of view are not a so good thing (from another point of view). In short: developer and publisher want more control over their product and Steam seems to be an answer.
But from user point of view? Following reasoning from Gamasutra article, Steam help users in having good things from DRM: social interactions, one digital shops, achivements, forums, multiplatform (on Mac too). It's worth ?
For some games yes: multiplayer, mmorpg, rts with intense multiplayer features. But for single player games, no, never. Require full time internet connection to work is a pain in a game like Skyrim (one random example :D)
What do you think about it ?