Don't know how other people feel, but I'm very dubious about using any rucksack on a bike.
Worried about taking a tumble and breaking back on contents of bag. Certainly on the few occasions I have used them, haven't put anything solid in them.
Having said that, a pack full of clothes might have a nice cushioning effect, provided it doesn't result in hyperextending (forced stretching backwards) too much.
Either way, I reckon a high-vis backpack doesn't look as, dorky (for want of a better, less american phrase (not that there's anything wrong with Americans, except perhaps Bush)).
