The good thing is that the plug-in works, although looks like the same functionality is available in the standard Live Preview. Maybe it is more usable for people with smaller screens, but with high-res it makes no difference.
The bad thing... Actually I needed the functionality provided by /?theme=my_custom_theme URL parameter for sending URLs with "test" theme to some people for approval. And these people are not interested to perform several steps like logging in, clicking something etc; they just want to click an URL from their email client and see the "test" version on their screen. By default this parameter is not working for "everyone", only for the logged in users :(.
I've found some workaround to make the URL parameter work after several hours of experiments, but it still not exactly what I need.