Manos is genuinely FASCINATING. Yes, it's bad -- that's kind of its thing -- but you know, people got together and made this thing.
It may not be as fun to watch as a Michael Bay movie, but it leaves a hell of a lot more of an impression. That's because it has character. It was made by people who gave a fuck.
I've repeated, more than once, Nathan Rabin's premise that a mere failure is WORSE than a fiasco, because a fiasco is interesting to watch. Similarly, Joel Hodgson himself once commented that the appeal of a bad movie is that you know what a good movie is supposed to look like and it's fun to see where bad movies fall short of that mark -- and that really, it's amazing that there are so many good movies because it's SO EASY to fuck things up.
Manos is a LEGENDARILY bad movie, but it's got heart. There's something inherently appealing about seeing a small group of people put a movie together outside of the studio system -- even if it sucks, it's still an accomplishment; they MADE something.
My girlfriend ran across some godawful Twilight spoof awhile back on Netflix; it was clearly put together by some college theatre geeks. We couldn't sit through more than ten minutes of the damn thing, but it left me thinking, you know what, Netflix is awesome; it lets shit like this get SEEN.
It's interesting going through the 0% Fresh movies at Rotten Tomatoes and contrasting them. Because I'd much rather watch Manos or Mars Needs Women than Jack and Jill or The Smurfs.