You can hear his unusual but very intelligent and sharp sense of humor in his lead work. Zappa was really ahead of his time in more ways than one. He had an incredible sense of melody, tone and tasteful use of effects that still remains unrivaled today.There is only a very specific section of Zappa I continually play over and over. Anything older than Overnite Sensation or newer than Joe's Garage really doesn't interest me.
In contrast, many (not all) of today's shredders and virtuoso's lack melody or whimsy in their playing.
Listening to Frank Zappa's music is not for everyone, but I have a feeling he will be re-discovered again and again by future guitarists. His playing style is almost impossible to approach.
He broke the rules, but in a great way!