Brilliant lengthy shooter; a bit buggy but better than HL2
I found it better than HL2 in every way: story, gameplay, graphics etc. Also, I never finished HL2 as it gave me motion sickness after a while (!) but Stalker did not. The only thing I didn't like was that sometimes your shooting misses and it may take several head shots but over a distance I guess that is realistic. Maybe I prefer tactical shooters more so prefer the one shot tactic.
I would not say it is really that much like a RPG as there is no character development, you just get an inventory that's all. If I played again I think I would download the mod that is out there that allows you to carry more weight as it was always a shame to discard the sniper rifles because I had no ammo for them. Would have been good to hang on to one until more of their (rare) ammo was found. Everyone likes an assault rifle, shotgun, pistol AND sniper rifle in their bag, right?! By the way, make sure you get the pistol that has been modded to use big rounds about halfway into the game. It is hidden in an underpass full of pipes and stuff (I think there was a task to go there for something...).
The game is a bit buggy but I only had 2 or 3 crashes in total; worth putting up with.