I love both the STALKER games out right now. I am currently playing through Clear Sky again. And if you thought the lighting on shadow of chernobyl was impressive, then you need to check out clear sky, the expansion game, which is actually a prequal in the story line. If you have a card that can run the lighting rendering at directx10 or 10.1, you will be in for a treat. Its the only game that has actually left me speechless in terms of graphics.
As for gameplay I think the most fun part for me is customizing my weapons loadout. I tend to collect a lot of weapons throughout the game and love clear sky for its ability to repair and upgrade weapons, including upgrades such as scope mounts, larger magazines, lighter components to reduce weight, etc.
I will say clear sky for me is more glitchy than SoC, but its probably because I spend more time in CS fooling around and straying from the main objectives. For me the most buggy part of CS is the emissions.
Some of the worst glitching ive had would include emissions happening 3 times in a row consecutively, or people who i try to carry out missions for dying before i can complete the mission. Because STALKER is such a "sandbox" game, where you have so much freedom, sometimes the amount of freedom the game has is too much. And this is seen when important characters that are supposed to stay alive die, etc. And when such people die it is often out of your control. Also, the mutants respawn too quickly. As far as crashing out of the game I only saw that when CS was totally unpatched. You MUST fully patch this game to get good gameplay.
All in all I think this game is a pretty damn good one. The story is great, although extremely complicated, and the widespread cries of crashing and such are generally caused when your computer cant handle the game's intense graphics. I myself, since patching clear sky, have never experienced a crash of any sort.
I would definitely play this game, and I would definitely buy clear sky. I would probably even get the 3rd one coming out this autumn called STALKER Call of Pripyat.
-Blue