Works great! It was very easy to assemble (less than 5 minutes) and surprisingly didn't add as much weight to the pi as I expected it to. Now I just need to figure out a way to put my fan back on with it!
Don't put the batteries in according to the marks inside the slots (it shows them as being parallel, + up on both sides) but instead put them in according to the marks on the PCB itself (+ up and - up). I hadn't noticed the other marks at first and thought my board was faulty when it wouldn't turn on.
The batteries it comes with are alright but not capable of powering the pi well if it's overclocked. The capacity also isn't that great so I'd recommend some better batteries.
As other reviewers have stated, it will not charge the batteries, so make sure you have an 18650 charger.
I would recommend using a short right-angle USB cable instead of the one it comes with because it sticks out quite a ways and I worry that it will get bumped into and potentially damage the connector on the pi.
Really, for the price it's a wonderful little battery pack.