Wow, this didn't make a lick of sense!! (spoilers)
In its defense I will admit that it has some originality, but "original" does not necessarily equate to "good." There were so many inconsistencies of plot and character motivation that after a while it just became ridiculous. Just a sampling of my observations:
1) Why did Hugh find it necessary to chloroform Jay and tie her to the wheelchair?
2) Why is Jay so matter-of-fact about Hugh giving her what amounts to an STD? What's more, why are Jay's friends so matter-of-fact about it when they finally meet Hugh/Jeff. If Jay were my friend, I would have torn Jeff limb from limb for what he had done.
3) Up until the entity makes its physical presence known and removes all doubt of its existence, why do Jay's friends never question her sanity, think she might be doing drugs, etc?
4) Why was Hugh staying in the rathole? I understand that he didn't want Jay to know his true identity, and that he wanted to set up "alarms" for the entity... but he could have done all that from in his own home, and just not brought Jay around his house.
5) The film is constantly rewriting the rules about what the entity does or doesn't do, what can hurt/kill it, etc. Why does it attack Greg with such ferocity but only pull Jay's hair?
6) After Greg is killed, no mention of him is ever made again.
7) If Jay cares about Paul, why does she have sex with him.
And this is a perfect segue to the most absurd part of all...
6) Paul and Greg can't wait to get into Jay's pants -- despite the fact that doing so will basically destroy their lives and they know it. I've heard of being p-whipped, but this is ridiculous!
The hosannas showered upon this film from the critics are a mystery to me. Perhaps it has something to do with its literary pretentions, dropping as it does a little bit of Dostoevsky here, a little bit of T.S. Eliot there...