A word on waterproof panniers. I don't know exactly what Arkel had to say about waterproof panniers, but for commuting and short errands they work well. I have a set of Axiom waterproof panniers and I don't need to worry about forgetting the covers or having to pull over in traffic to put the covers on.

For longer use, however, like touring I wouldn't use them. Hot weather, ouch They're fine up to like 1h maybe and then they get really hot inside, and outside too btw, there is no ventilation at all.