If you haven't played this already – and you probably haven't --, it's a must. Preferably in a darkened room with headphones. It remains one of the most memorable of all first-person shooters, and with superb gameplay to boot; it remains a bit of a mystery why it wasn't a hit. Inevitably it looks a bit dated now, but on the other hand that means it runs exceedingly smoothly. This game pushed the original Unreal engine as far as it could go, but with better character models than many Unreal-based games.
The game has a very strong storyline and extremely good writing (thanks to Clive Barker, though it's quite different from his books). The main theme is, obviously, horror, but with more of a focus on creepy atmosphere than on jump-out-of-your-seat moments (the latter are provided mainly by combat). The different phases of the story also have very different atmospheres: there's a crypts-n-vampires section, a grim-n-gory-bad-guys section, a weird-fantastic-otherworld section, and so on. One part of the story near the start of the game does lag a bit – after you leave the mansion and are going on a quest at a monastery – but it picks up again very nicely quite soon.
The gameplay is quite unique. Combat involves one hand wielding a range of real and magical weapons, while spells are cast with the other hand; the result feels like a foreshadowing of Elder Scrolls IV: Oblivion.
In short: a classic. Definitely a must-play. It did in fact win an award for “best game no one played”.