If you're the sort of rider who thinks that fording a waist deep river in the pouring rain is all part of an ordinary day on the bike, then this is the bag for you. Ortlieb are a German company well known for making waterproof travel and expedition bags.
Many cycle messengers also use their quirky roll top model to keep packages dry in transit. This is, however, their first proper backpack and it is fully waterproof. No really, test it, fill it with water, zip it up and nothing leaks out, which means under normal circumstances water cannot get in.
Detail wise this bag is fully loaded. The bag has a hydration hose port, which is fully sealed so even with the hose exiting the bag, it's still watertight. Inside the bag has a pouch pocket and zip compartment pocket and a pouch for your hydration bladder.
On the outside the bag has one long vertical compartment (also fully watertight), twin side pouches, a zip out mesh helmet net and an LED light loop. If you never let bad weather stop you riding, but you hate wet gear, this bag is in a league of its own and worth every penny asked.