Enjoyed it and shocked (possible spoilers)
First of all let me begin by saying I totally understand and agree with the possession theory.
If you see the movie again with this in mind it makes perfect sense.
I saw 'The Rite' the other day and found it boring and predictable, in your face possession with no mystery or suspense, not for me anyway. There's also the issue of the younger priest trying to explain the possession as a psychological phenomena, which is what we see in 'first born.
In 'First born' the possession is not obvious and the way it is presented is very subtle and 'real life'. On the outside it looks like the woman is suffering from severe PPD, which is what we would all assume if this really happened, but in my opinion all the clues are there to suggest possession.
I enjoyed the movie a lot for its subtlety and the ending really shocked me.