How bad is it? Unknown.
Should you see it? Unknown.
I was going to put this in an addendum, but the passing of Jerry Lewis this week makes this seem an appropriate time.
In the summer of 1970 (maybe 1971), my local theater showed all of Jerry Lewis' films from his solo career, in order, once per week. This gave my mother some needed time off and I became a pint-sized film critic. His earlier films were simple, easy, broad comedy, but there was always a dark serious note in the background. As the years passed, his films became more introverted and deeper in concept, more difficult to like. By "Don't Raise the Bridge, Lower the River," (1968) he was no longer funny and it seemed he was exorcising inner demons through film. "Which Way to the Front" (1970) was a disaster and it's themes point the way to this final, unreleased film of this period. It is quite likely that the film, while technically competent - even artistic, is unenjoyably bad.
Thirty minutes of the film have been released online and a script is available, but few have seen the best rough cut that Lewis had in his personal collection and donated to the National Archives registry of film (with the proviso that it not be released for some time). The official story is that the film got lost in a legal battle that Lewis never completely understood; the unofficial one that the film was unreleasably bad. The truth is probably that there was fear that the film would ruin the reputation of Lewis because of its handling of controversial themes. The plot follows a clown put into a Nazi concentration camp, who leads children to the gas chamber; Roberto Benigni managed to make a decent comedy of similar material as "Life Is Beautiful," but all reports are that "The Day the Clown Cried" misses the mark by a wide margin.