The bag is simple, large, and good looking. The attachment is easy. But it is not rigid, and that causes the bag to wobble and move side to side. I have an Ortlieb bag which is quite a bit smaller and has a small bracket that affixes to the seat, as well as a simple plastic subframe running along the center which combined solve the wobbling problem, but the Apidura does not have that. I wish it did, as I like all other aspects of this bag.
The bag is very well made and indeed waterproof. For that I give it 5 starts. However, the usable volume is definitely not 14 liters. I put a trash bag in it and filled it with water to check the volume, and 14L fills it to the very top with the bag wide open. With the roll-top closed (2-3 turns), I estimate the usable volume at about 10L. This is unfortunate because I was counting on using the full 14L for weekend touring but it simply will not work for me. If you really want a bag that can handle multi-day touring then I recommend getting the 17L model which is significantly larger.
Just took it on a 2-Day ride on the Katy Trail in Missouri. Check it out if you haven't been on this great trail and it is now linked to the Rock Island Trail. 267mi! After I figured out how to correctly tighten the straps, it was very secure and I hard noticed the pack behind me. I crammed a lot in there and didn't even max it out for a two day non-camping trip. There was no rain, but I felt comfortable with the quality of the material that my stuff would stay dry. I plan to add the front pack for a longer ride in the fall.
Ordered one for me and a friend so we can haul some gear on our rides. We end up in places where it would difficult getting a ride home should anything happen so we go prepared with tools, extra clothes and a DSLR on occasion. Pack works very well and can hold more than it looks. Haven't gotten it wet yet but the material does seem very waterproof.