Very impressed in the quality of this seat pack. When fastened to the bike the seat pack is very firm and steady. Would highly recommend seat pack support strap which can be purchased as an option. Planning on using the seat pack for overnight bike trips with my fat tire bike.
This bag is incredibly well made and definitely waterproof. I would give it 5 stars except for one important issue. I carefully packed my bag to have the heaviest items at the bottom, and upon mounting it never showed sag, just a nice angle upward from the seat. I did a test ride on some pretty rough gravel and no problem. However, on my trip I was on gravel and had numerous cattle guard crossings and it began to slip down and would bump the tire requiring me to stop and re-tighten. Fortunately I had some Voile straps(highly recommend for their versatility!) and was able to "boost" it up. After getting home and doing some research, it turns out Ortlieb makes a "Seat Pack Support Strap" to address this very problem. They suggest the problem is from improper packing, but honestly I think this support strap should be included. This was on the smaller bag, so I can only imagine it would be necessary on the larger one. The slipping occurs in the attached webbing straps. Ortlieb's price is $xx for the support strap. Definitely get it or plan to use a Voile, which might actually be more durable.