I have one, but don't use it much.

The backpack zips onto a "mounting plate" on your rack(you fold the pack straps-like some travel bags to take on a plane). The plate can be easily removed (no lock mechanism), so I ziptied&tied it to the bike it is on. The bag itself is more of a "holder" for a kayak type dry bag (the dry bag is rather heavy-so I used one I had from Arkel). The bag can easily hold 12Kg, and depending upon if you add extra mounting ties to the plate it should hold-If the zipper doesn't give out. If you do decide to get one, watch your heal clearance, as it is a rather large bag-clearance is fine on my MTB/Winter Commuter with a Jandd (very long) rack.

Overall I would not rate this product very well.

If your in the Boston area I also have one new-never mounted, I'd sell for 35$ (I won't ship-sorry).