I saw the original band in 1968 in NYC - I've always thought that there was a loose edge to them that no suceeding Zappa band has matched, a much closer tie to older rock and roll, and more of a feel for the old stuff, which made the NEW stuff even more interesting.
one intersting thing about Zappa, to me, is that, though he claimed the opposite, he was a real lover of old fashioned rock and roll, and understood how great that music could be. The night I saw him, he introduced Sam the Sham, who was in the audience, brought him up on stage, went into a huddle with the band, and produced a letter-perfect version (with Sam singing) of Wooly Bullly -