Even Siskel & Ebert liked the movie :)

Siskel even couldn't guess the plot twist. They also criticize Willow for "being a Star Wars ripoff", in the video.
https://www.youtube.com/watch?v=Lf89pUUBk78&feature=player_detailpage#t=335

Too bad it made no money whatsoever, though.
https://en.wikipedia.org/wiki/Jack%27s_Back
Box office $492,519 (USA)

Then again, this is a perfect VHS rental or late night TV movie.