I saw a woman eat it when her clip on seat bag came loose and jammed between her rear tire and the frame. Stopped her bike almost instantly, but due to the magic of physics, her body kept going forward. Ouch.
I've come to like the Serfas road bags, mainly because they're decently sized, but very narrow and don't shred shorts.
Mine holds a spare tube, multitool, Co2 and inflater, money, a few other things.
http://www.serfas.com/product.asp?ProductID=106