The Purge is an abstract idea given form, a kind of voice for a psychotic fantasy. Gratuitous violence served cold.
The good. Great idea for a movie. Well done suspense. A few nice twists in the story. Perfect ending.
The bad. Subpar acting for the most part. A few logical flaws, but not too many.
The ugly. Characters without any depth.
The result. For thrill seekers who like novel ideas. Others should probably abstain.
On a personal note, I was not able to suspend my disbelief concerning that future. To me, this is completely impossible. I even find the idea untenable. To me, what makes us human is our ability to go beyond our animal instincts.
I think this flick would have been much if they had explored more of this night instead of focusing on a single house. It might have given the opportunity to experiences unintended consequences or other interesting concepts.