Ortlieb have a long heritage of manufacturing waterproof bags and panniers to the highest standards. This is the Flight, their new waterproof rucksack. It's available in two sizes - 22 and 27 litres - and both are large enough for a full day in the saddle. The material is tough and hardwearing PS33 fabric that ages very well.
The main compartment features zippered pockets and a sleeve for hydration bladders, with the hose routing through a sealed port that takes a maximum 10.5mm diameter hose.
Outside there's a vertical front pocket and two side neoprene stash pockets for drinks bottles. All seams are 3D welded and the main pockets are zippered with two-way TIZIP waterproof zips. The back includes strung mesh coupled with a breathable harness featuring waist and sternum straps.
In more stretched-out riding positions we found the tall profile of the bag knocked the back of our helmet, and £120 is a lot of cash, but if you do lots of bike commuting or wilderness riding, where keeping kit dry is essential, then that's a price worth paying.