Know the limitations and you will be alright. Package arrived with no documentation. But the website has enough to get you going. This UPS will basically allow you to safely power down the Pi should power be lost or just cycled. But you have to power off the unit and turn it back on in order for it to charge the battery. In the 2 days I have done testing, it doesn't appear that the battery will charge with the hat turned off or after power has been restored. You have to power cycle the hat in order for it to start charging again. If you have the hat plugged in, it will keep powering the Pi. Your battery will just read 0% and the battery voltage will be really low. That's right the unit will keep draining the power from the battery after a power cycled. The hat has to be power cycled. It looks like the battery that came with the hat will run for about 2 hours. That is with 2 wlans and a network cable plugged in. Running Kismet in the background. Used the example python script from the website and was able to do a shutdown when the battery got low. For me this will work great as a portable penetration tester.
I have seen only one other hat that claimed to do more. Same features with an RTC. But that one has really low rating. No Documentation at all and it cost 3 times more than this one.
For me the perfect UPS would be the linux compatible APC USB smart ups in a hat. That you can program to restore power after the battery has reached a set capacity or after stable power has returned for a length of time.

It is a portable power supply and will switch to battery power if the AC fails but it doesn't go back to charging the battery when AC returns. A button must be pressed to restart the charging process. Useless for unattended sites.

This arrived DOA. It appears to work with LEDs suggesting it's charging (and their driver reports 100%, while the 4th LED is still blinking). When power is interrupted, it fails. Zero functionality as a UPS. I've contacted the seller and depending on the amount of hassle involved I will consider a replacement. Otherwise, I'll return to Amazon and pursue a different product.

Previous comments about build quality seem fixed as it was very sturdy assembled to my Pi 3.

Edit: DOA issue was battery wires were reversed at the connector. The seller responded to my review and my message promptly and genuinely wanted to solve the issue (for me and the future of their product). They have offered to replace the board to ease my concerns with the battery condition after it was in use with the polarity reversed. Will update my review again after it arrives and I'm set up. So far, I'm surprisingly pleased with both their effort and resolution. As far as the board is concerned. It looks like the board I have now does require the power cycle to regain charging of the HAT after power is disconnected (pressing the button). That's unfortunate, but not a deal breaker for my application. I'm just protecting from rare brownouts.

This works great! The only downside to it that I'm running into is that the GPIO pins are too tall, causing it to not fit into a few cases. The only other tweak I'd make is that the socket connecting the board to the battery, in most cases, forces you to have the wires exposed out the back of most cases. If it were on the oppose side of the power port, it'd be perfect.

Seems to be working as planned so far. I've got the tools installed on the Linux OS, and they work. Does charge the battery and power the Pi at the same time. Also, I am running this on a Pi Zero, without any troubles.