I hate backpacks.

[Crankin]

The only time I commuted with a backpack, I hurt my back. I am also 5' 1" and the messenger bag was HUGE on me. I commuted using my Camelbak, with the bladder taken out; it was just enough room for some papers, make up, and my lunch. I would bring my clothes to work on the days I didn't ride.
My "commuter" bike is so heavy compared to my road bike, I chose to ride the road bike, instead of the bike with a pannier. I could never seriously ride with a backpack or messenger bag; I suspect there's a big need out there for someone to design a messenger bag that would fit short women. A WSD messenger bag!

[Melalvai]

I hate how hot and sweaty my back feels when I'm biking with a backpack. I don't do it if I can help it. I have an enormous rear basket, a grocery bag pannier, a kitty litter pannier, and a trailer with a huge plastic tub on it. I never have to wear a backpack on my bike anymore! I just bought a new and bigger rear basket, and gave my merely "big" basket to my daughter so she doesn't have to bike to school with her backpack on either.

[colorisnt]

Did the commute with two panniers and my Trek trunk. Awesome! Got up the hill fast and arrived NOT covered in sweat or feeling like death.