I like the xootr crossrack (
http://www.xootr.com/crossrack-bike-rack.html) plus an arkel utility pannier (although you can use any pannier). I haven't used this for touring (I use conventional front and rear panniers), but it works well for grocery shopping around town.
The achilles heel of all of the one-bag systems is the weight limit; I haven't found one that holds more than 25 lbs.