At first I thought this was a funny, dark, mindless thriller. I was wrong. It is smart. Very smart, and amazingly crafted. Several story lines weaved into one story, with characters that'll make you laugh and just hate. Even the hateful characters will make you laugh.
There's a lot to say, but I can't think of what to say.

It's a horror. That's a given. The horror aspect gives a feeling of paranoia, it really digs into your bones when shit gets going.
There's a mystery involved as well. This part really helps the story to keep you guessing as to what's going on and why? It adds the needed depth to help push the story, to give it a plot instead of just making us paranoid about something living in our head.
I actually thought I had it figured out at the beginning after the surgery. Then when that went to hell, I thought I had it figured out again, then THAT went to hell and well you get what I mean. Maybe a little predictable but it still keeps you guessing.
Very awesome story.