It was ok actually. It obviously wasn't going to win any awards, but it had a good deal of action and the special effects weren't bad.
The first person-shooter sequence was awesome though; pinky demon!
I think they could have based it more on the first and second game, with loads of monsters rampaging about and even including the Spider Mastermind and the Cyberdemon battling it out!
Anyway, Doom: definitely not the worst film ever, just average.