Its a messenger bag, and back problems arent an issue when carrying one loaded with stuff.
Panniers have their issues, they dont carry well off the bike, they get filthy when riding in bad weather while mounted on the bike, and they typically dont hold much. Most panniers Ive seen, it would take at least 2 or 3 to equal one medium sized messenger bag. On top of that, panniers need hardware to mount them to a bike, meaning, switching bikes means switching the mounts too. panniers as noted above, also affect the bikes handling bigtime. Given those traits, messenger bag works better cept on really really long trips.