I think Mark Hamill said it best when he said (and I know I'm paraphrasing slightly), "That's the great thing about the Darth Vader character. You don't need an elaborate backstory to him. Everything that you need to know about him is right there in the original movies."

Also, I'm reading the old 1978 Star Wars book "Splinter of the Minds Eye" and in the re-issueing of the book a few years ago there is a introduction by Lucas that plainly states in his own words "That his story was too big to fit in one movie, and that he had to break it up into 3 parts. The original trilogy, the further adventures, and the back story, making a total of 9 films."

Heh, and he says that the "Sequal trilogy" is just a figment of the media's imagination. That lying basterd.