Yes it's because of the alledged asset stealing. But I think that it's more likely all 3 games just purchased them from the same 3rd party developer. I'm sure time will tell
I've never played Doom3, but if you've played HalfLife2 I think a lot of it feels very similar to the Ravenholm section. That had me physically jump a couple of times the first run through of the game. Playing STALKER gives me a permanent tingle in my spine, because you don't really know what's going to happen next. Which is a good thing.
The (frankly amazing) AI fights amongst itself, so you never know when you get somewhere whether it will be allies or enemies in control of checkpoints or roadblocks. So extra tension if you're not on your guard at all times.
The environments are very well designed, although the underground bunkers do all look a little similar (not really a shock though). The choice of weapons is good, although because you have a weight restriction, you can't carry them all. And where I am in the game, ammo is becoming very short.
The idea of factions I've not yet explored, since this is my first run through, but apparently you can chose which side you want to ally with. I ended up with one simply because it was the only option available to me at that time and I wanted the mission
I've almost hit 50 hours of play and covered very little ground twice (other than physically running back and forward to speak to people). I'm actually thinking of heading back out of the area I'm in to pick up replacement armour, and if I can make it past the guys trying to kill me I should be able to - the main missions tell a linear story, but you can stop in the middle and go back to do other things if you want. It's nice and you-specific.
I heartily recommend it. Heartily. And my one piece of advice is this - save OFTEN!