My thoughts on the back protector largely follow those in my front protector review. The product truly does seem invisible, and MediaDevil as a company are very friendly, their customer service is oustanding!
I felt I should simply share some advice with regards to applying the back protector. I eventually managed to get the protector on with no bubbles or trapped dust largely by following the instructions given by MediaDevil. However, I would like to emphasise that you should make absolutely sure your back protector does not touch the silver ring around the camera.
My first attempts at using the product were made without this information (the instructional video is more tailored to the front protector) and I couldn't seem to avoid bubbles appearing around the camera. An extremely helpful email from MediaDevil told me that my difficulty was likely a result of the protector touching the silver ring. On my second attempt, I made sure the cut fit perfectly around the ring, and zero bubbles appeared.
I simply love the finished look. There's something very satisfying about the fact that my 4S has lost none of its visual appeal and yet is fully protected from scratches! Recommended. Just follow the instructions carefully, the cut is very precise around that camera!