Bags of fun! Non-stop intense action!
Think of it as an extremely good combination of Halo*, Quake 4 (the entire game just reminds me so much of it) and Vexx (the puzzle-solving aspect).
The brilliant graphics, combined with the music, and immersive storyline just kept me playing it for hours on end. Although not particularly challenging, this could be considered a good thing - the relative easiness with which you dispatch your enemies allows for a rollercoaster of action, and keeps the story flowing non-stop. This gives it the distinct feel of not just a game, but a likeness to a film in some ways.
All in all, an excellent piece of work - congrats to 2k on a great game.
*I say Halo rather than Halo 2 because of the excellent storyline, which Halo 2 did not possess, and the excellent music, which Halo 2 also did not possess.