Ortlieb Bike Packer Plus (
http://www.bikebagshop.com/ortlieb-b...us-p-1010.html) $180; more expensive than most but they are waterproof, spacious and quickly attach/detach from the rear rack with little effort - simply pull up on the handle and the latching mechanisms open and allow the panniers to lift clear of the rack. I've bicycled through several downpours and everything inside was dry.