This movie, like Casablanca, has lines that are memorable and come to mind in daily life at odd times in a funny way. "You'll have to kill Me before I die!" or "Ahhh, Prison has reformed him!" (in the movie in reference to Yellowbeard (aka Graham Chapman) supposedly raping with his clothes on since he used to "take them off in the old days" as Madeline Kahn puts it. Something like this could fall flat on it's face with boorishness, but done by masters of comedy, they spin gold with hay.
For me, like many British comedies, it gets funnier the more you watch it the first few times, and holds it's charm long afterwards. I've had friends watch it the first time not thinking much of it. Then giggling a few times the next 2-3 times. Then belting out laughing the 5th+ time. It's a movie that has "in" jokes that you become a part of with time.
This movie is a mixture of cast members from "Young Frankenstein", Cheech & Chong, and a few Monty Python members. The screenplay writing is quite rash. "I'll pin your tits to the table" or "It can't have been you, I'm sure I killed the last one I raped". What makes these lines even remotely funny is a normally stoic Graham Chapman delivers a rare outrageous performance and delivers them with rare gift of phrasing and timing. Such a natural talent.
It gets poor ratings on IMDB and from Ebert, and even Cleese supposedly said it's "one of the worst 5 movies ever made" but I have always had a bizarre sense of humor and for me it's my favorite comedy. Pirates, Sexual Humour, outrageousness, and for once, a Pythonish skit with an actual plot!
And when people try to dismiss this below-the-radar-jewel, just remember, it's never more dangerous than when it's dead!!