The first one is masterfully done, in the 'Blair Witch' sense: it's all meticulously scripted and yet the acting and editing feels so natural that you could easily fool someone into believing that it's real. Is it anything I'd watch more than once? No (even though I bought the DVD for the making-of features), but having seen so many crappy found footage films, I respect Fred Vogel for executing the concept well.
The second and third ones, though they up the gore considerably, really aren't very good. The acting is noticeably worse and it tries to integrate a storyline which feels too forced and unnatural. In short, it falls victim to the same things that plague bad found footage films. The worst thing a found footage film can be is unconvincing because then the entire premise of the film falls apart, and 'Mordum' and 'Penance' are such examples.