Although alot of people don't understand the fact that this film was shot on a 16mm film camera, it had it's flaws. But imagine using actual film to shoot a movie and ending up with a result like this, children of this time cannot imagine the hard labor to make something like this!
I myself am a massive fan of Ti West's style; normal people doing normal stuff, ending up in not so normal situations. Today the crowd needs big motion, crazy plotlines and exceptional characters, with off course a shit load of CGI special effects. West proves that this is not needed in order to entertain a crowd.
Although it's really slow paced, some scenes were too dark, the atmosphere of the film in general was really nice. Acting was just enough to believe most of the film! Great first try of West, and watching his further films, you see he progresses as a horror filmmaker and sure fixes the flaws "The Roost" has!
One thing to add, is that Ti West is a master in telling certain stuff with only sound! And he proves right! These days when something fucked up happens, like a kill for example, it's a raw shot and the crowd sees the gore. What actually makes something scary, in my opinion, is hearing how someone dies and leave it for the crowd to imagine how cruel the kill actually was!