This game is fantastic. It's the most atmospheric game I've ever played, it's great at making you jump and some of the later levels are downright disturbing! Read emails and listen to audio logs - not only will you find codes and so on for doors, you'll also pick up on a lot more of the story - if you don't read the emails and listen to the logs, you'll miss out on a lot of immersive background information. The game is also nice and long, you won't be finishing this one in 5 hours (stand up at the back there, CoD MW2!).
One last thing: don't spoil the game by using any stupid 'duct tape' mods so you can have the flashlight stuck on your gun. There are several moments where you don't feel in control, that you don't feel like the typical Uber FPS character - these moments are destroyed by 'duct tape' mods. Sure, it's not realistic that you couldn't find some duct tape to stick the torch to your gun, but remember you've only got one flashlight, so when you switch weapons, you'd still have the same problem. For me, it's a superb game mechanism that ID have used here - it's a genuinely creepy experience to have switch from torch to weapon.
Finally, if you played this when it first came out and was frustrated (as I was) by low FPS, now's the time to give it another go! Your modern PC should chew through this like a chainsaw through zombies! Oh - one other thing - play about an hour at a time, otherwise the repetitive nature of the game will set in, and that'll spoil your Doom 3 experience.
Have fun... and roll on Doom 4! In the meantime, check out Quake 4 - another superb ID game.