I still think you should get Stalker. It's pretty much my favorite FPS of all time, and trust me I've played a messload of them. Some people don't like it I think cause it's pretty tough at first, the instructions blow, and it just takes a little while to 'figure out'. Once you do though, its a blast. I've played through to Chernobyl countless times at this point, there's so many ways to do 'stuff'. It's got huge, open environments, accurately modeled weapons, a fairly simple 'rpg' system, great storyline, multiple endings (only one 'right one' though), incredible immersiveness, very decent graphics if you have a good machine, and some really freaky parts. For $20 you just can't beat it in my book, if you are into great single player campaign FPS's. Never tried the multi-player though.
However, I would buy it on a real disc though, not through Steam. If it crashes too much it tends to get corrupted, so it's nice to just have the dvd handy for reinstalls. You should also seek out and install the latest patches, plus the 'Float32' mod for it, makes it look/perform much better.