Haven't tried one but I'm intrigued. No rack/pannier BS such as rack mounting, heel clearence, broken racks/panniers etc. Tons of space, supposed to have smooth ride, no special bike required, not expensive, stable handling, works on dirt, useful when not touring. Supposed to weigh less than 10 lbs., rivals the weight of racks and panniers.
If you already have a mountain bike all you have to do is get one of these and your good to go. That's a mighty cheap way to get into touring.