R means over 17, at least according to Wikipedia.
In any case, I saw it last night. It started off slow, with a bunch of killings in the normal way, sort of. It seemed rather disappointing, for reasons I won't go into here. I was thinking that this was going to be a lame, killing-spree movie. But as it progressed, the plot improved, and I actually really liked it. I liked how everything, including the killings at the beginning, all came together.
One thing I didn't like: it was too friggin' dark!