OK, when you said "to whip a camera" I imagined taking taking a cam out of a tank bag, and snapping a picture without getting off the bike, but you had in mind something different.
I agree, you can't quickly take the things out of fully waterproof duffel bag, but you'll need to make a choice between convenience and waterproofing. If the Wunderlich was fully waterproof and easy to get into it could be worth the asking price. But on the bottom of their description they say that the bag is not fully waterproof.
Recently I had to ride a miserable day in a downpoor. I had this bag http://www.thewaterproofstore.com/dpd2gr.html
strapped to the rear seat. The main compartment is rollup, and it has packets with waterproof zippers and flaps over the. By the end of the day one of the packet had a half an inch of water in it. I wouldn't risk it with Wunderlich.