My advice is always this: Don't buy panniers that attach using just hooks and a bungee. These have an unfortunate tendency to fly off when you hit bumps, and/or shift around on the rack.

Either get ones that have hooks that latch on (Ortlieb, Arkel, and some others) or ones that have conventional hooks but a much stronger connection to the lower hook on the rack. I use Inertia Designs panniers, which have the latter system, and they have never fallen off. They have also been extremely durable over many thousands of miles. This is what I have: http://www.amazon.com/Inertia-Design.../dp/B001OS43DM

A couple people suggested the Nashbar waterproof panniers. I had a pair of the large size, and they basically fell apart under similar usage to my Inertia Designs panniers, which have held up great.

This is one of those times when it makes sense to just spend the money up front.