I've spent all kinds of $$ on Chrome, Crumpler, etc. to find the *perfect* bag to ride with.
Turns out all I needed was the free bag from a vendor at Interbike.
Buy a fancy laser and all the cat wants is a grocery bag to crawl in to.
Its light, holds a change of clothes + flat repair with room to spare. I don't notice the straps and it doesn't heat up my back. I did find out the hard way that its not waterproof though.