The Purge films are the kind of lean, clever exploitation pics that you could imagine Roger Corman making in an earlier era. The concept isn't original at all. James DeMonaco is dusting down and repackaging ideas that have been used in countless vigilante or zombie films.
What makes The Purge: Anarchy work is its taut time frame (once a year, the Government allows murder and criminality to go unpunished for 12 hours only), its political edge and its cleverly chosen mix of characters cast adrift in the big, bad city on the one night when they want to "stay safe" at home.
DeMonaco cranks up the tension very effectively in the early scenes, just before the "purge" begins, when all the killers in their Leatherface-like masks are beginning to assemble.
Once the bloodletting is underway, the film becomes increasingly repetitive. Frank Grillo is the good-hearted action hero who has ventured out to avenge the death of his son. En route, he bumps into, and tries to save, some innocent civilians caught up in the carnage.
The originality and suspense have been used up long before the final reel, but one guesses this is a franchise that will run and run.