My ex-girl had one of those harshell packs, they suck IMO. Any backpack with straps around your waist are uncofortable on a bike.
I ride with a Kriega, and anybody who is serious about riding with a backpack on a motorcycle should get one. It's the only backback that's comfortable when leaned over, and stays in place better than anything else too. It's doesnt' work for hiking or anything else, but that's not what it's designed for. It has a harnass on your chest holding it it place and taking a lot of weight off your shoulders, looks kinda like a parachute. And all the straps and wires are held in place, so nothing flaps around on your back at high speed (150 mph plus)
I've used it on a 2000 mile trip, and it's perfect.
Expensive, but you'll thank me after buying it. It will also last forever, so it's cheaper than a cheap backpack.
http://www.kriega.com/