The only level where i was constantly on the edge of the sofa was Chesire, *shudder* just thinking about it now scares me, the worst part is that it is almost completely silent, save for the dripping of a pipe.....
As far as FPS go there are scarier games out there. Biohazard, for example, maintains a fairly chilling atmosphere throughout, with plenty of scripted creepiness - the first encounter with a houdini slicer, the female spider slicer that talks to you throughout a level, etc. The world in RFoM is bleak but not terrifying.
There are a couple of levels where you'll be wondering what's around the next corner, but as for enemies that actually jump out at you? There are only a few moments.
We captured time so time made us all hostages without mercy.
Same with me with the menial thing, I just got the game yesterday, started the first Grimsby level with the conversion centre. Saw these menials loading stuff into this thing. I walked near to them and shot 1. The other one started lumbering towards me, I thought it was funny because it was so slow and then it jumps on me and starts eating my face. I totally freaked and just yelled at the top of my voice.