I agree with the reviews that complain about the pack shifting or even coming open during a ride. I had this problem for the first couple of weeks. I finally figured out that I needed to pack it with the bare minimum--two CO2 cartridges with one connected to nozzle, one tube (700cx28), one Parktool tire lever, one Parktool GP-2 patch kit, one Parktool I-Beam mini tool. Arrange all contents into roll as flatly as possible. Fold roll loosely so that the both side of the velcro strap line up exactly. When securing to seat rails, ensure that the velcro strap and the padding used for the BOA wire are pressed together. The friction between the two surfaces is key to keeping roll fixed in place when riding. Once I figured this out, I had no problems but feel Silca should have given these instructions rather than learning through trial and error. This should make sense if you give it a try.
Love this bag. Tons of room, compact, and easy to get on and off. During a repair, I can pull off and get to everything with the bike upside-down then easily reattach and be on my way. I converted all my bikes to this bag.
I use this seat bag in ebike touring to hold an extra 11# battery and some rain gear etc., that I can access quickly. It holds very well to the seat post even around a cushioning spring (Body Float). I have generated a small rip in a key strap on an older one, and the main seat straps are very difficult to attach, but I've solved that problem using some long zip ties which are easier to attach. The front velcrop straps hold very well and the rear straps work pretty well at securing the opening.