An incident a couple of years ago involving a deep rut, a separated shoulder and a 40 mile ride home left me with a dislike of carrying anything on my back when riding ... until 2 weeks ago. A week long job monitoring a DofE cycle expedition in the Brecon Beacons had me searching for a new waist pack - bum bag type thing, it had to be waterproof, lightweight and have enough capacity to carry a days worth of gear but I couldn't turn anything up so gave up looking. Whilst buying some new dry bags for the same trip I happened to spot the Gourdon dry bag / backpack hybrid things at Alpkit. I decided that for the money if I didn't like it I could just use it as a normal dry bag so ordered one.
I went for the smallest 20l one and it's great. Firstly, it weighs next to nothing and it's fully waterproof (as you'd expect a dry bag with straps to be) but it also has a bladder pocket, 2 mesh side pockets, an external bungee for lashing extra stuff on and a removable foam back stiffener which makes a handy seat. I used it again this weekend whilst doing a first aid course. Packed in it were, waterproof trousers, Montane winter weight jacket, 2l of water and my butties plus a few bits and bobs. There was still plenty of room to spare and joy of joys it didn't give my shoulder any grief!
I reckon if you're still pondering how best to carry your gear then you could do far worse than buying one of these. They're available in 20, 25 and 30l versions and various colours so there might be one to match your eyes. Go and have a look here http://www.alpkit.com/shop/cart.php?target=category&category_id=295 and see for yourself. P.S I'm not just saying this and yes I did buy my own.