I finally installed STALKER today to see how good it was and am so far very impressed. The outdoor graphics put Oblivion to shame. On my setup I have it running at 1280x1024 with detail on max with dynamic lighting and it runs very smoothly with no slowdown at all. The outdoor textures look great and there is no lose of detail at distance.
The game is also great fun so far and for me I think many elements show what was missing from HL2. Much more non linear too.