Originally Posted by
CastIron
Ortlieb. They have an back harness for 'em. Have your cake and eat it too.
I would like to second this man's notion. I have a set of Ortliebs on my rear rack and I absolutely love them. I have put all kinds of things in there and have stuffed them to overflowing. They are also waterproof, which I know is not a criteria. But, it also means that you could fill the bags with a bunch of ice and some beer and go for a Sunday stroll around the park and have a beer or two wherever you go. Oh, yes the Ortliebs can haul a huge amount of weight. Make sure that your rack can take that sort of abuse. I had to change racks because my old one couldn't carry the heavy loads that were now capable with my then newly arrived Ortliebs. I have had mine for about a year now hauling fruits, veggies, books, cat food, gallons of water, dirty clothes, cooking implements, etc.
The system that CastIron mentioned is a harness system where one of the Ortlieb bags gets converted into a backpack. I have never used the system so I don't know how good it is but I do know that the Ortlieb connection system onto a bike rack is one of and maybe is the best system on the market. It takes me probably all of 5 seconds to load a bag onto my rack and about 1 second to take it off.
There are a number of other choices out there and I don't think you will be disappointed with anything that the people here recommend. They have been through it all and know what they are talking about.
http://www.ortliebusa.com/CartGenie/prod-31.htm
http://www.ortliebusa.com/CartGenie/prod-75.htm