In general, I loved it. I loved how often you'd just see the people in the background, but it didn't specifically focus on them; I didn't notice him until a few seconds later, and it startled the crap out of me.
The three biggest things I didn't like about it: They WAY overdid the 'OMG she was just there a second ago' thing, the husband was just GONE most of the movie, and the ending was disappointing. They just... stabbed them. That's it.
I still really liked it, though. I liked how it didn't try to explain any of it; it was just some messed up people looking for shits and giggles.
Also, it's GREAT for scaring a girl into getting closer to you...