If you don't want to have to change batteries . . .
This is a fairly specialised bit of kit and I appreciate that not everyone will have a use for it. I bought it to power a preamp for a piezo bridge on an old archtop guitar. I didn't want to cut into the body so I wanted to fit all the controls under the floating scratchplate.
The unit is wider and longer but also slimmer than a conventional 9 volt battery. It fits well beneath the scratchplate - glued with a double-sided tape that seems very secure - and still sits clear of the guitar top. I would suggest that you need you need to be comfortable with guitar wiring to fit it, though. The charging, via USB, works well. Simply plug it in, leave it overnight, unplug it when the light says it's fully charged.
I have no complaints about the unit, it does everything I hoped it would. I would give it a total of 5 stars except I think they're a bit too expensive to justify buying for general use.