A decent FPS, not spectacular but not bad either.
The story takes some time to develop and each character has a unique background. Not essential to the core game but certainly adds another dimension to the overall experience. It's not great, but notable for how few games even try to build in compelling storylines. Max Payne did a far better job, but Unreal II made a nice effort.
On a C2D Extreme X6800 with 4 GB RAM, 2x8800s, and Vista x64, loading times were absurdly long. So it's really painful to die. I didn't find any way to skip past the in-mission cutscenes, so save after each one. Your character seems to move very slow. I guess it's because he's wearing substantial armor, but it was frustrating how slowly he ran.
On first blush difficulty struck me as high on several missions, but after a few attempts (and consulting the GS GG for the missions I didn't particularly like) it's really not that bad (at Medium difficulty).
My general impression of this game is pretty good, despite the fact that I hate Defend This Area type scenarios.