I just adore the Rutles! I have "All You Need Is Cash" on DVD-- it's time for another spin.
As you saw above, George thought it was hilarious to the point of sending himself up (his Rutle counterpart is Stig who is "thumped" by Big Valerie). Eric Idle reports that John and Yoko enjoyed their bit of ridicule as well, which Eric was nervous about, as he had cast Yoko as a Nazi ("She really has a great sense of humor" he said). Paul was reportedly rather uncertain about the whole thing, but Linda thought it was so fantastic he came around to her viewpoint. Or so the legend goes!
Honestly, the more you know about the Beatles, the funnier the movie is, because lots of jokes are based on detailed knowledge of the original Fab Four.