It's fun. It's fun because shotgunning zombies will always be fun. You couldn't screw that up if you tried. Hellspawn and hot lead are one of natures great pairings. Like salt and pepper or fish and chips. They were simply meant to go together. However... (you knew there was going to be a 'however', didn't you? Actually I bet you were expecting a 'but'. Well, there's no but here, buddy, so HAH!)
However, I have a problem with the way the game is set up. It is a pretty close copy of the old school doom games, but with ratcheted up eye candy. The problem I have with that is that it could be so much more. I thought serious sam sucked for this reason, it was nothing more than fragging the same old crap over and over and over ad infinitum. But still, this isn't too bad. Like I said, Ordinance + the undead = family fun. The main problem I have is that the game strives to create an atmosphere of tension and dread, which it does, but it does it too much.
How 'too much'? I hear you ask. (you should really stop talking to your computer, your mother is getting worried) Well, the classic (because it freaking works) method of properly building tension is to give someone a tense moment, then a relaxed moment, then a tense moment, then a relaxed moment, and keep them going one after the other, faster and faster and faster until you reach the dramatic pinnacle. Doom 3, on the other hand, gives you one relaxed moment. The moment you start the game. From the instant you're sent on your first mission to go find the missing scientist (about two minutes in for those that haven't played yet, no spoilers here) the tension starts up. And it DOESN'T. LET. UP. That's a bad thing. Not because it reduces me into a quivering pile of terrified goo, quite the opposite. Because too much tension building without release and the mind simply goes 'nah, I don't buy it,' and shuts down on you. Now no longer are you frightened when something jumps out, you just say 'wah, another imp, bang bang, next?' The great effort that's gone into making all these tense moments is wasted, because you simply can't remain tense for that long. The fear is replaced by disbelief. The shock when something pops out of the ceiling is wasted, because crap has been popping out of somewhere for the last eight rooms and six hallways and honestly, your brain just doesn't like that much SHOCK and SURPRISE and FEAR!!!
No pacing, is what I'm saying. RE had its safe rooms where you saved the game and relaxed for a bit and knew that you wouldn't have to worry about beasts jumping out at you. Then when you went back to the dangerous outside world, you were mentally refreshed and ready to be scared up even more. Doom doesn't have anything like this, and I think the lack of it really harms the game.
And that's my rant on doom3.
But, to reiterate, it's not that I don't like this game. I just feel it fails in a few areas that it could have really shined in. But still: firepower + minions of satan = fun.