I haven't used one or seen one around here for quite some time. I could imagine them screwing up your back in a crash though. Might want to keep that in mind when picking one out.
When I ride to work I take a kind of big backpack (it's really normal size but it feels big when it's flapping in the wind) because it fits my work shoes and clothes. Though, when I just go riding around I try to take my camelback which is pretty small and fits my wallet and a sweater in case it gets chilly (I wear a mesh jacket).