Finished it on normal difficulty. Only got stuck a couple of times, principally with the Amoebas and that level where you have to close off the two entrances to that station while being swarmed by nosalises (incidentally, I played this in the dark at about 2am, very intense). God, the environment is so engrossing, certainly I felt like it was a very well thought out story.
I love the sections where you are on a mobile train and you can see the wires on the tunnel walls - reminds me so much of the London Underground, where you can see the same thing out the window. If the Bastard wasn't so bad, I'd have used it more - I love the magazine on it sliding through while you fire. Gotta replay it again, try at the lower ranger difficulty perhaps.