Got this for the XBox a while back, didn't really have much time to play until today, when I was home sick.
Pretty damn good overall, if a bit slow to start. It's lame that they artificially slow you down by not allowing you to have your flashlight out at the same time as your weapon. And the game really drags as you wind through the first stages. Trudging through Alpha Labs, you wonder if this will ever be fun. Your persistance will pay off, however, when you get to Delta Labs and points beyond... way beyond.
Complaints? Well, aside from the flashlight thing... (Yeah, the duct tape mod for the PC is great, but those of us playing on an XBox are still screwed.)
I felt the weapons were a bit nerfed in general. Take the rocket launcher (please). Considering how rare ammo is and how slowly rockets seem to travel, the thing should really kick tail. Instead, it takes TWO shots to down even the relatively weak cacodemons. The splash damage seems to be nearly nil. I seem to remember in the original Doom shooting a wall and watching two or three nearby demons explode into chunks. That'll never happen to you in Doom 3. Ever. The shotgun is useless unless you're right up in someone's face AND you make a headshot. Even then, it's surprisingly weak. You'll be lucky to kill the most common enemy, the Imp, with a single blast. If you're scared and shaky, it may take three or four close range shotgun blasts to send them back where they came from. Good aim is absolutely crucial.
On the other hand, the chainsaw is freaking miraculous. All but two non-boss enemies go down in less than a "one one-thousand" count if you can keep the blade in them. The BFG is pretty damn good too. There were a couple of places where I used it to pulp two hellknights with a single fully charged shot. You don't get much ammo for it, but that's to be expected. Actually, you're going to be very ammo-poor until the very last level of this game, so play accordingly. When they start to give you a whole lot of ammo, worry. They're just fattening you up for the slaughter... The Soul Cube is very cool. One recommendation there: keep it charged, and only ever use it on Archviles. There were a couple places in the game where they would send a hellknight or two after you, and then put an Archvile in the next room. If you expended your soul cube charge on the knights, you're most likely going to end up dead at the Archvile. In fact, the Archvile's only real weakness seems to be the soul cube. They're very tough and nothing else really seems to hurt them much.
Most of the enemies seem to have a weakness of some sort for you to discover and exploit, come to think of it. Wraiths will die in one good shotgun blast, though they're fast so you'll have to be good to hit them. Imps take a bit to charge up their fireball, and it's reasonably easy to dodge, giving you time to hit them with the necessary two good shotgun blasts. You can blow up the Revenant's missles in his face with the plasma gun. The tentacle zombie troopers are too stupid to notice that you just ducked, and will swing right over your head. The hellknights are slow and will fruitlessly chase you around pillars all day. Etc.
Graphically it's a fantastic game. Of course they made it far too dark - on purpose. The environments are really cool looking, if intentionally cramped and crowded. Oh yes, those of you who are squeamish about blood are going to absolutely HATE the later levels. All the PDA displays and computer consoles in the game are really neat, and I enjoyed that aspect of the game as much as the rest of it. Some of the audio logs and laserdisc mini-movies you can pick up and watch had me laughing out loud. (One easter egg late in the game is an email you can pick up in a part of the base controlled by the demons. It details the proper procedures for human sacrifices: "Candles must be placed tallest in back, shortest in front - NEVER the other way around!") Later on in the game you also get to some really shiny dynamic environments, and those are awesome as well. I wish there had been more of that, actually.
Length is just about where I like it. It took me about 20-25 hours, but I'm a seriously paranoid player. I always keep my back to a wall and check every corner. I'll take out the flashlight, look around a room from the door, then stow it and walk in ten feet, keeping my back to the wall. Flashlight out again, looking around for likely spots that demons will teleport in to. Then back to the gun, creep another ten feet, back against the wall. Repeat ad naseum. But if you had the heavy balls to run through this thing, I bet you could do it in 10 hours. (I haven't heard of any speed runs, but I'm sure they're out there...)