I adore it too. Just saw it again very recently and was impressed by how not dated it felt.
F for Fake is pretty much required viewing for anyone who is a magician, documentarian, journalist, FBI forgery analyst, or Orson Welles impersonator.
In a conversation once, I had someone say to me that, in F for Fake, Welles "is saying that 'it's impossible for us to ever know what is true and what is not.'" Boy do I think that's a misreading of the film. I think the film makes a series of playful assertions, which often get doubled-back on—but nowhere does it assert that actual empirical reality is somehow inaccessible. Rather, I've always felt that the film kind of says the opposite—that there is such a thing as a fact, and that humans are quite good at hiding, distorting, and forging facts to fool other humans; some (like Elmyr... or Welles) have it down to an art. Yeah, something like that. Point is, F for Fake kicks ass.