Was going to start a new thread on this but saw yours so posted here instead.
I used to use a rucksack if cycling a long distance and / or planning to stop where I would need to lock my bike up, but have recently been suffering with a sore shoulder. A fortnight ago I went on a 20 odd mile ride and planned to stop at a canal side pub for a bite to eat and a quick beer so had my rucksack containing my butties, my lock and a change of top in case I was sweaty and needed to change to avoid cold (I carry a spare tube, pump and tools in a small saddle bag, but that's too small for my lock and food).
As I rode I felt the pain in my shoulder increasing and over the next couple of days it crippled me. I seem to have a trapped nerve in my neck / shoulder now and it is sheer agony, disturbed sleep and unable to ride my bike. I have seen a physio and a chiropractor who have both said they would strongly advise against using bags that put weight directly onto the shoulders, the physio said a back pack with a waist strap to divert some weight to your waste would be better.
I know back packs are widely used so it seems a bit drastic to suggest nobody ever uses one again, but they are capable of causing injury so best not to pile too much weight in them. When my neck / shoulder gets better, I won't be using one again. I am going to get one of those bags you can attach to your crossbar instead.
Good luck.
