I still remember when I bought this game for the PS2, I was in middle school. To this day its atmosphere has stuck with me, very bleak game which was downright scary for my younger self (definitely horror for me, at least until a certain point), especially with how defeatist the first part of the game feels, before transitioning into a lone wolf rabbit hole. But it was also quite fun to play. A strong singleplayer experience that manages to be atmospheric both when you are alone and with allies. Now looking back (or better said: looking at gameplay videos) I realize it's not a perfect game, but I would totally buy a reliable version.
This and F.E.A.R. molded me into craving horror FPS experiences that feel creepy, hopeless and yet a blast to fight your way through.