I think one of the main reasons people seem to have not taken to this episode is the fact that they seem to be trying to focus on the monster when the focus of the episode was always supposed to be Vincent himself. In a way, the monster was just an aspect of his character and deperession given physical form. The episode was all about character and fairly distanced from the usual structure of a DW episode, so I can see why some might not have taken to it all that well. I personally thought it was a refreshing change from the usual 'mystery monster, discover monster, run a bit, doctor fixes things' formula and the ending was brilliantly written; as Steven Moffat put it in confidential, the one thing the doctor cannot save people from is themselves.
Just as an afterthought, I liked the little mentions of Rory during the episode, possibly forshadowing that Amy probably will remember him eventually (explosive emotional finale anyone?).