Burton

Ideally a backpack should have an adjustable harness comprised of wide, padded sholder straps that distribute the weight across the back, a sternum strap to keep the shoulder straps together and a padded hip strap so that weight is carried by the hips as well as the shoulders

Unfortunately such a harness is a little bulky so most 'convertable' panniers have thin shoulder straps rather than a complete harness and are completely unsuitable for carrying anything more than a few lbs for short periods of time.

Lambert Cycles had an amazing model on the market a few years ago under the Voyager name that not only embarrased anything else on the market - it was half the price. Unfortunately it didn't sell well and was discontinued. Talked to the guys in product design at Lambert last year at ExpoCycle because customers gave been asking for it so there's a small chance it might resurface.

I was lucky enough to pick up a couple myself and they probably do more duty as a backpack than as a pannier - they're that good. But the next best thing would be a bonifide backpack dropped inside a simple pannier of similar size and then loaded from the top.