I got stalker on Wednesday and have been playing a fair bit of it. The game reminds me of Deus Ex mixed with some morrowind. They made a really atmospheric world and some of the underground levels are quite scary. I've not yet got to Chernobyl itself but it won't be too long before I do. You could easily spend 30+ hours just doing side quests or killing pratically anybody you see.

The highlight for me was when I accidently angered the Duty faction and had a huge firefight in one of their outposts, neutral Stalkers in the zone were getting gunned down left and right in the crossfire. Ended up getting a nice rifle off one of the expert stalkers. :)