I have had several Lowepro shoulder bags for a couple of years -- mostly for carrying equipment to sporting events, band competition, etc. Not heavy use but also haven't babied them -- always good luck with them: rugged, good construction, no defects.
Wanted a back-pack and ordered the Lowepro Flipside 200 Digital SLR Backpack. A little smaller than I wanted: camera section holds Nikon D200 w/17-55mm + one additional lens. Upper section can hold additional lens, flash, etc. Once my wife started to use it, though, she has been quite happy with it and uses it all the time. I've since ordered the Flipside 300 and the combination of the two covers all the gear we need. Flipside 300 is noticeably bigger both in the camera section and the upper area -- great for transporting gear.
Both have appropriate adjustments and have been very comfortable for long periods. The 200 is best when you don't need much, 300 when you need to take more. Highly recommend both.