I've seen both films as well as Overnight. Yes, the first film is extremely fun to watch and has some interesting WTF moments. The vigilante aspect of the Saints is definitely a guilty pleasure in many ways. The second movie is also very well done. I don't want to spoil any of it for you, but the new one has quite a jaw-dropping ending which was certainly designed to leave room for at least one more sequel.
"Overnight" is another matter altogether. It was quite a disturbing "sausage" look into the making of the first film and the descent of favor that the writer, Troy Duffy, experienced while executing it. He claims it was a smear job, although it is difficult to reconcile that statement with the actual events that were caught on tape behind-the-scenes. He really did seem to go out of his way to piss off the people around him trying to help make this movie. Apparently, even Harvey Weinstein stopped returning his calls - definitely a "you broke my heart, Fredo" moment. My only advice on this particular film is to get it and draw your own conclusions about it. All documentaries, by definition, are biased to varying degrees, despite whatever the author's best efforts may or may not be.
However, despite all that, Duffy did seem to dig himself out of the ashes somehow to create a new and quite enjoyable film. I didn't find the new soundtrack to be as inspiring as the first, however, even going so far as recycling my favorite track, "The Blood of Cuchulainn", but the rest of it was quite good.