My grade: A-, I really liked it.
The worst part was probably the last scene, the 'proto Alien' or whatever. It felt like a cheap way to tie in the film to the Alien series. Not needed at all, and ruined a pretty good last shot.
The best parts were the quiet and cerebral parts (more than I expected, much appreciated). I liked the big issues tackled.
The biggest plot hole for me was David's lack of interest in what had happened to Shaw. So he orchestrated the whole thing as an experiment to see what would happen - except that he showed no interest at all in what Shaw removed from her body.
It really does seem like there is a longer cut out there that addresses this and other issues. I would LOVE a 3+ hrs cut.