Under The Skin is not for everyone. The film has a slow pace and as the io9 review notes there are long stretches of not much happening. If I had watched this in a theater I probably would have been squirming in my seat about halfway through. But I'm glad I watched it. Spoilers ahead.
One thing I kept thinking about in the first half of the movie was how far I would ride with a strange woman in her vehicle (particularly a serial killer special like that van). My alarm bells might not go off at first for a short ride to where I needed to go. But the personal questions and seduction would definitely make me suspicious. I'm not the guy that sort of thing happens to. I would like to think my horniness wouldn't override my good sense but I can't say for certain.
People have commented on Scarlett Johansson's flat acting in The Winter Soldier but it's effective here. Her alien-in-a-human-suit is mimicking human behavior without really understanding it.
The biggest revelation for me was discovering that Scarlett Johansson is left-handed. When she picked up the fork with her left hand to try the cake I actually paused the movie and went online to verify that she was indeed a leftie. Is that in anyway significant to the movie? No, but it was still an interesting tidbit of information.
A lot of things are left unexplained and open to interpretation. That's fine. A lot of what you take away from the movie probably depends on what you bring in. I suspect men and women see Under The Skin in different ways.