As a casual gamer who will occasionally really get into a title if it is particularly addictive, I have to say I am somewhat let down by DOOM 3. I was not expecting a simulator, nor an RPG; in fact, just the opposite. I was expecting something akin to the original two games... that is, lots of fast-paced action and rapid discharging of the ammo. What I got, it seems, is not so fast-paced, but more of a "creep along until another one jumps out at you" kind of deal. The PDA system is innovative, but doesn't do a lot for me in practice.
I downloaded a copy to "test drive" and only played to about midway through the second level, and was already becoming bored with the repetitive nature of the engagements with the enemies. It does not help that the framerate is "marginally playable" at best on my P4 3GHz/Ti4200 config even at 640x480/low, despite certain hardware sites that would have had me believe otherwise. From what I can tell, the multiplayer aspect of DOOM 3 is mediocre at best... I know that wasn't supposed to be the focus of this game, but as someone who rarely plays through a single-player campaign more than once, it becomes a critical factor in my decision to purchase the game. I think I'll stick with UT2004, which is approximately 1000x more engaging in that arena.
All in all, considering the exceptionally high price, combined with the fact that after a few hours of "trial play" I am not all that impressed, I think I will be deleting the game and saving my money for now... however, I am fairly confident this will be a game I do purchase 6 months or a year down the road when I have more capable hardware and the price has come down a bit.