Yay, it works!
I discovered this product on Kickstarter, after it was funded, and was delighted to be able to add one to my recent Amazon order.  It was annoying and disappointing to find it wouldn't stay on; didn't know why till I checked the 1- and 2-star reviews, in which the reversed polarity of some magnets was mentioned.  Quick and easy to fix, however:  I removed the li'l magnet, re-inserted it in the correct alignment, and ta-da! it stays in place rather than vigorously shooting off.  Much more effective than my home-kludged contraptions; happy to have and recommend the gadget.
Rating: 4 stars