At first I was pretty sceptical about the service.  I decided to try out Geometry Wars off it though, since it was only $4, and the game seems pretty cool.  Well, that worked fine.  A little while later, they've got The Orange Box on sale for $37.50, and 10% off pretty much everything (Boxing Week sale this was).  I can't refuse OB at that price, and I'd been meaning to pick up The Longest Journey for a while, so I got both those bundles.  Since then I've added the GTA bundle that got released (aside from a weird known glitch with the GTAs on dual-core systems, those games are great - the glitch is that every time you start the game, you have to pull up task manager and set the GTA process to only use one core, or it acts like an old DOS system with the Turbo button on lol).  I still want to get the Civilization pack (I normally dislike that style of game, but the demo has me addicted) and there's a pack with all the old Wolfenstein games plus a couple of old DOS classics I used to love like Commander Keen in it.  I'm trying to refrain from spending anything else until I at least beat a couple more of the games I've downloaded though.  >_>
At least I can scratch The Longest Journey off my list (now on Dreamfall: The Longest Journey, its sequel), as well as Portal (although I haven't done all the challenge maps yet).  I'm most of the way through HL2 as well.  As for GTA:VC (the GTA I decided to start with), I'm not sure how 
far through that I am . . . I just love messing around in it.  

  Oh yeah, I also want to get the DS version of Geometry Wars now.  That game is addicting even with only the one level available.
So yeah.  Steam turned out to be a pretty good portal into PC gaming for me.  And a little hard on my pocketbook.  
