I'm gonna break down this review into 3 parts: The graphics, the beginning, and after 30 minutes.
The Graphics: It should be noted that I run a pretty powerful machine. 2.8GHz P4 (800MHz FSB) with 1GB RAM (800MHz FSB), and my vid card is an ATi 9600XT. That puts my machine well into the requirements, and my vid card performs about the middle of the pack (performs better than the 9800, worse than the 9800 Pro). I played the game the way the game recommended: 640X480 Medium Quality.
The graphics are gorgeous! The game is a touch choppy, but definately playable. There are points where the game would pause to start up a scripted sequence. Now, I have good raw power on my machine, so I was expecting choppiness maybe in the video card, but not the actual machine (coulda been from loading up the data on disc... couldn't hear the CD ROM). Graphics are definately beautiful in this game as expected.
The Beginning: The game is just like HL. You start off normal, then 'things go bad.' The funny thing, though, is the 'scariness' of the game is in the 'normal' setting. You have to go through underground Mars where crazy stuff happens (like lights going out in certain rooms, etc). It really makes for a spooky atmosphere. It started to remind me of the first time I played System Shock 2 (the only game that scares the crap outta me, constantly). Speaking of SS2, they nabbed the idea of the PDA from SS2. You can hear audio logs from other people through it, making for an even better environment. Then bad things happen. Some cool effects like zombies flinching the part you actually hit. In fact, there is one part where a body is hung upsidedown from the ceiling that I gleefully punched just to see him swing.
30 minutes into the game: Well, 30 mins into the game is well after 'bad things happen', but this is when I realized something different. I wasn't scared anymore. The game was... well... it was just doom. Zombies and demons pop out, you kill them. Nothing scary about that, in fact, I felt a little bad compairing Doom3 to SS2 in my mind. Honestly, it is Doom with a plot (well, a plot slightly better than the original doom), but it gets monotonous quickly, because the gameplay is just like Doom. You get access to a new room... go into that room, kill a buncha things, you get more of the plot and access to a new room, go into that room, kill a buncha things....
The game, unfortunately, gets old after an hour of gameplay. Sure, its gorgeous as hell, and they worked really hard to make a good plot, but the gameplay is still the same old same old. Worth the $55? Maybe once a few mods pop up (a la Halflife).
Maybe I'm a bit spoiled from playing Warren Spector's FPS/RPG genre-breaking games, but Doom3 just didn't feel like anything new except for the graphics. Sorry John Carmack. You are a brilliant coder and can do wonders with graphics, but your games are all the same gameplay.
To sum up, I didn't 'hate' the game, just got bored with it after a while. I still may continue playing it, but just to flesh out the plot and see the new monsters' graphics.