Immersive but repetitive nonetheless...
But seriously, the atmosphere in the game is amazing. The ambient sounds, voices, occasional screams and heartbeats in the walls are quite unnerving, especially when played past midnight in a dark solitary room.
The major downside of Doom 3 is that the action is pretty repetitive and after about 5 levels of pretty much the same demons invariably springing up from behind you - it gets uninteresting. The game is straight forward, meaning, one can’t stray from the path that’ll lead you to the next target since all of the doors miraculously break. There are no side stories (which would be very un-doom-like, I gather, but would add a nice touch to the game :)).
The storyline was very remotely reminiscent of Stargate, with the ancient tablets, corridors covered in writing and the artifacts abandoned by some long extinct civilization - all for Homo sapiens to find.
Starting off on the Martian base of UAC corporation, the player is sent on a mission to find out what’s going on at the lower levels (scientist went missing?) of the base where all Hell breaks loose (quite literally). During gameplay one can find PDAs of recently deceased co-workers and read their private mail as well as audio logs. The latter become more and more disturbing as the game progresses revealing the reason for Hellish creatures taking over the UAC.