I experimented with it just recently. I found it very easy to use - you just upload scanned images of your sketches, and create links between them, then you get a link to access the page for testing. The problem I had was that the dimensions of my sketches were different to the screen size of my phone, and I couldn't find a way to get the image to fit in the screen correctly. I tried modifying the screen dimensions in POP and everything else I could think of, but in the end I gave up as I just couldn't be bothered redrawing all my sketches (or digitally manipulating them all). I would give it another go, but next time I'll ensure I use the correct dimensions for my sketches (and test it first).