I cleared 2 of the games out of the 3. I found one of them somewhat buggy. I mostly play at the most difficult and sometimes just under the hardest mode. With all games, the higher the difficulty the more the game somewhat cheats.
S.T.A.L.K.E.R.: Shadow of Chernobyl Cleared it - Awesome
S.T.A.L.K.E.R.: Clear Sky Too buggy, didn't clear it
S.T.A.L.K.E.R.: Call of Pripyat Cleared it - Awesome
Faction Wars was pretty buggy... but it would have been nice if that system was extended to CoP and polished up some.
Clear Sky was my favorite. There is a fan based mod for the faction system that fixed up a good portion of the bugs and fleshed out some of the unfinished aspects of it. I'd say with the fan made mod the faction wars aspect fells like it's about 90% complete. Without the the patch it feels like it's about 60% complete and just slapped into the game. Who ever made it did a pretty dang good job. The faction war mod, I had fun restarting and joining up different factions to see what groups would play nice and who wanted to kill you....then you work on taking out camps and letting your own faction setup in those camps, great times.
SoC atmosphere was amazing. It had some glaring issues when it first released and the pin-point accuracy bad guys had with grenades was irritating as hell. You could be 50 meters out, hiding in a building and taking potshots at bad guys....then all of a sudden the on-screen grenade icon comes up and immediately BOOM! it explodes because the grenade cooked during the long flight, but doesn't actually explode until it hits the ground or you....I think the issue was eventually fixed in a patch. I played through the vanilla version a few times, got the true ending once and a couple of the alternate endings. I found you could "cheat" by utilizing pine trees; if you hid on the inside on some of them and shot out at bad guys, they wouldn't even take notice as you killed them...but that issue may have also been resolved with patches. I loved trying to head to small army roadblock nearby the STALKER camp at the start of the game....it was hard as hell to get up close and kill them all with a starting pistol and sawed off shotgun.
CoP was my least favorite - it was fun, the story was okay and some of the atmosphere from SoC was brought back. But it felt too linear compared to CS and SoC.
All in all, the series was very enjoyable. I probably put 200+ hours into all the games (nearly 120 into Clear Sky once I found the faction war mod).
Hopefully STALKER 2 can live up to what the series has already given us.