I ordered one of these beasts the other week. I was finding my battery was running out of juice before the end of a day of normal use, so needed something sorting.
It cost me about 30 quid from a Facebook ad, and took about a fortnight to arrive (from China).
Charged it up last night and today, after 8 hours of use, my phone battery is still at 100% 😎 It's a bit chunky, but it does actually make the phone feel more robust and less likely to snap.
I have no affiliation with Floegrip, I just thought i'd let you know that it's a great bit of kit.