On the first hand, the game is frightening. Yes, it is. I didn't believe it before playing, but once you get immersed in the game, it really becomes so. For the best experience, turn off your lights, rise the volume and, of course, play alone. If you like horror games or movies, you'll certainly enjoy this one.
The ambient at the beginning of the game is neat. You have few ammo, you discover new monsters, you hear screams through radio transmissions... Easy to get "involved". As you progress, you'll learn about the game's plot basically through other people's PDAs, which hold mails and voice messages. These are also recorded in a way to increase tension, as in some way they outline what will happen soon you go on.
On the graphics area, it is very good, even though it requires a good amount of resources. I have finished it at high quality and there certainly are a lot of details in the images, plus a lot of neat effects. As regards sounds, they are very well placed to create a horror ambient. I think some background voices were much better in the English version (the one I started playing) than in the Spanish one, but it has been so long that I cannot tell for sure.
Let's now see the negative side: the game is too repetitive and too long. That's why it has taken me so long to finish it: I've played it in four runs, because after each one I was too bored to continue playing. Well, being too long wouldn't be bad if there was more variety in the levels... in fact, I'm not sure it's that long, but it seemed so. For example, the Delta Labs seem infinite.
Also, once you get to the Hell level and onwards, you find so many monsters that it becomes not so frightening. It is more a "shoot everything that moves and run" than "go slowly to see what may happen" as is at the first half of the game.
Summarizing, it has more pros than cons, so it's worth it. If not, I wouldn't have just got the expansion pack, Doom 3: Resurrection of Evil!