Fun at first but gets boring and tedious later.
First of all, some elements of the gameplay mechanics don't make much sense. For example, pummeling a zombie with the blunt end of your flashlight does a lot more damage than shooting them with your pistol. Also, the AI of some of the monsters is not very impressive (crouching down and hiding under stairs works a little too well!). Finally, we've all read about the flashlight issue. In the real-world, a little common sense would've avoided that one!
My single biggest gripe about the game is that it gets very boring after a while. Monsters tend to follow the same pattern: 1) you walk into a room, 2) a monster jumps out of nowhere to ambush you, 3) you gun it down, 4) you walk into another room and start the whole process over again. Since the game is pretty low on plot to begin with, this is a particular problem. Finally, the single player mode can be summed up in one word: LONELY. The game is considerably long and you go through the vast majority of it by yourself. To make matters worse, 98% of the game takes place in small rooms and long, narrow hallways, creating a feeling of lonesome claustrophobia that just doesn't go away!
Doom 3 is a stunning game from a graphical standpoint. But the boring gameplay of the single-player mode combined with the shallow plot and gloomy mood make this a hard game to play through to the end. You'll almost thank yourself to quit!