I finally won Doom 3 (Xbox) on Nightmare, and what a challenge. Some parts of it were brilliant, other parts were too frickin' hard and frustrating. There are some parts where you simply don't have enough room to move around with such low health, and other times you're completely out-classed against the monsters you're up against.
Overall though, Doom 3 is an excellent game. It's probably the best looking Xbox game I've ever seen, and it's got an excellent atmosphere and sound track. I do think it could have been trimmed a little shorter though, as parts of it did drag a little (which was already trimmed a bit from the PC/Mac version), but this can be over-looked.
Anyone who has an Xbox, PC, or Mac and is looking for a sci-fi/horror first person shooter with cutting edge tech. and classic gameplay driving it, you can't go wrong with Doom 3.
I'm hoping id Software decides to port the PC's expansion set, Resurrection of Evil, to Xbox as well. They were thinking about doing that as a premium download via Xbox Live, however there hasn't been any official word yet.