In the same vein as Scream and Cabin In The Woods, Behind The Mask does a great job of being a reasonably self aware horror flick. The documentary aspect is a great twist to add to a story that could feel overdone if you aren't careful, and it helped elevate this movie to a great level.
I enjoyed the various moments the movie referenced other killers as well. It was especially amusing considering the cameos from Kane Hodder and Robert Englund.
The end came as a surprise as well, it made the movie feel worth the watch.