No, really. Sinbad is easily more entertaining than Plan 9 from Outer Space, which is more commonly considered the best bad movie ever.
I won't give away all the details that make this movie so laughably awful, but I'll mention a few that I hope will convince you to find and watch this film. (You can read a more comprehensive review at www.rinkworks.com/badmovie/m/sinbad.of.the.seven.seas.1989.shtml)
John Steiner hammimg up the role of Jaffar, the villain. In particular, he exclaims one two-letter word that must be heard to be believed. You'll know which one I mean when you hear it.
All the music is done by one guy on a synthesizer, a total of about five different sound effects are used in all the fight scenes, and all the lines are DUBBED OVER. It seems the filmmakers didn't have the ability to record live sound on the set.
All action scenes are done in slow motion, and this is made painfully obvious by Lou Ferrigno's flapping, overgrown, oiled man-pecs.
However, all these shortcomings pale in comparison to the dialogue. I'll conclude with a snippet:
Sinbad: Wait a minute. There's nobody here.
Poochie the Dwarf: There's nobody here.
Sinbad: I just said that. That's the point.