I have some throw-over motorcycle panniers that were sitting in a
cupboard for a few years now, and it occurred to me that they might be
re-purposable.
I chucked them onto the rack, and with one occy strap, had a pretty
solid rig:
http://www.flickr.com/photos/z0s0/tags/touring/
They're big, but not heavy, and are mega-expandable. Expanding them
wouldn't be good for the wind resistance profile, but at least the space
is there if you need it.
I loaded them up with some heavy stuff, and took it for a quick ride,
jumping some kerbs for good measure. Everything stayed in place and the
handling wasn't particularly screwy.
I think I'm sold!
Only drawback I can see is that the mass in them sits a bit higher up
than regular bicycle panniers. Time will tell if that's a problem,
although having carted a slab of beer in a basket on the back, I'm
confident that's not a huge problem.
Cheers,
Jules