I've read the preceding comments and they pretty much tell the story of why this is a classic Jerry Lewis film. However, I think one of the reasons I love this one so much is that it also captures a feel for what life was like growing up in the fifties (as I did). In many ways it typifies the fifties mindset, but also exemplifies the entertainment of the period. Things were so much more family-oriented. Sex wasn't non-existence in films, but it was more often handled tastefully. Language was wholesome and the humor didn't depend on "shock" (such as the name of Navin Johnson's dog in The Jerk).
I find the segment in Hill Valley of the fifties in Back to the Future especially enjoyable as Robert Zemeckis does a great job of re-creating the era. Watching Rock a Bye Baby, which is authentically of the era confirms that. (In fact, the court house and town square in both of these films appear to have a striking similarity to one another.)
Although the story owes a lot to The Miracle of Morgan's Creek as inspiration (and even gives Preston Sturges credit), it really has its own unique flavor as well. Like a son who bears a resemblance to his father but also has his own personality. In many ways I think that Frank Tashlin has improved upon it. The subplot with the sister and its resolution are wonderful additions.
Over all I think it is a great film and can't wait till it's available on DVD (Is anybody listening?).
