Dill Pickle seat bags are hand-made by Emily O'Brien of considerable fixed-gear fame. They are thoughtfully designed and beautifully crafted of nylon and cordura. Fully lined, with an interior zipper in the liner for access to the outer shell for repair or stashing a few emergency buck, keys, etc., they are as waterproof as a bag gets. Hers stayed dry through 3 1/2 days of rain during PBP 2008! Sizewise, they are somewhere between a Carradice Nelson and a Barley - roughly 30cm x 17cm x 24cm, and MUCH LARGER when the top expansion flap is opened. These bags are notably lighter (~1350g) than a canvas bag of similar size, with 4 outer pockets plus a nifty sleeve pocket (brevet card holder) in the top flap. Reflective piping is on the sides and back. Expensive? Certainly. Worth it? You bet!