For the price this is very disappointing, I bought a clear plastic case for a third of the price that is about its equal. As others have said, it is very slippery, more so than the actual S4 back itself, e.g. I have a very slightly sloping plastic surface next to my chair that I tend to put my phone on when not charging and when this case is fitted it will immediately slide off yet will stay in place with the standard S4 back. Additionally, on my S4 there is a raised bar across the speaker port that means when placed on a flat surface, between that and the raised rim around the camera, the speaker isn't covered and can be heard quite clearly. But with this back, even with the slight raised rim around the camera aperture, the speaker output is effectively deadened when placed on a flat surface. If it was £2 or £3 it would be OK with its faults, but at this price definitely not recommended.
The only positively thing I would say about it, at least if you don't change your case often, is that it is a very very snug fit. However, if you do change your case often, whether for fashion or other reasons, perhaps changing SD card, this is a pig to get off it is such a tight fit. I had to insert a blunt edge between it and the phone's case and work it around to get it off. Using anything sharp would likely have marked the phone so much effort was required.