I have just bought the uNu Unity battery case for my Samsung Galaxy S4 (August 2014) and I am very impressed with it. Coming from an Exogear Exolife Battery case (which I already like - bought it last year around November), the uNu Unity Battery Case is a significant improvement. I got back my full functionality of the USB port, which is compromised when you use a traditional battery case that piggybacks your phone on an external battery via the microUSB port (the battery case USB port is usually not data-capable - charge only). I also noticed a significant increase in battery life. With the Exolife Battery case, I use my Galaxy S4 for around 10 hours on a typical day before switching on the Battery case to charge the phone (around 15-20% charge remaining). The phone gets hot while charging and I only get it charged up to around 60-70% capacity before the battery case runs out of juice. Obviously having the phone on while charging from an external battery pack is not the most efficient mode of charging, so I get an additional 6-8 hours of moderate use out of the phone with the Exolife battery case.
With the uNu Unity battery case, I was able to use my phone for 19 hours (more than adequate to a typical day) with the battery life at 60% after that time. The last time I was able to use a smartphone with this kind of battery life was my old Palm Treo 650 which lasted 3 days on a charge for me...
Overall, I am very impressed with this battery case. The uNu Unity battery case works like an extended battery without the ugly backplate that typically comes with extended batteries. And you get to keep your NFC functionality since the NFC antenna of the Galaxy S4 is part of the battery. Also, unlike the Exolife battery case, the NFC signal is not blocked by the external battery part of the case so it remains functional.
There is some creakiness to the case when you handle it, but when I removed and re-installed the battery case, I managed to fiddle with it so that all its clips engaged properly, making the gap between the backplate and the new bezel flush, thus minimizing, if not eliminating the creakiness of the case.
And compared with the Exolife Battery case, the Unu Unity battery case is shorter and thinner as well.