Look, this is easily the worst of the four [REC] movies. It also gets a lot of venom. That said, it isn't a bad film. If this was a stand alone film, it would be a perfectly serviceable zombie flick. It's a novel setting, several of the characters are fun. There aren't any gaping holes and some fun plot points.
It isn’t a classic, it doesn't break new ground, but it is watchable and enjoyable as a stand alone film.