Initially, it DID work for me. Only I could see any changes to the site. I had my site open on another computer next to me, constantly checking to see if any changes made their way across from my possible new theme to my current theme. And they did. When I made some minor changes in the WordPress theme customizer, the background colour of my current theme was changed to that of the theme I was testing. I then spent a few frantic minuets trying to correct the problem, as my nice blue-white gradient had been replaced with a cream-grey that clashed horribly.
In other words, I could not install, tweak and test plugins without breaking the visitor experience, as advertised. Hence, 1 star.