I hate backpacks.

[radacrider]

+1 on the messenger bag. I find it much more comfortable. My son, this year, switched to a messenger bag for school and he likes it a lot better than his backpack. (He also figured out he does not have to carry everything all the time.)

[Cataboo]

I use a deuter race xp - the back is mesh and curved away from my back - so the backpack is not actually against my back and doesn't make it too hot.

I also keep the waist belt tight and loosen the other straps - so the weight of the backpack is mostly on my waist.

I've cycled with messenger bags a few times. I hated it.

[colorisnt]

My bag is a deuter too. However, I just can't ride with it on. Fits in my homemade panniers though! The messenger bag thing also was a fail. I am 5 FT even and have no torso and it just gets complicated. Tommorow I will give it a shot with everything in the panniers so I can use my core again. I am so glad to hear I am not the only one!

[lph]

I like riding with a backpack, but only my very narrow one. It's basically just an extra pocket for my Osprey hiking pack, a narrow almost unpadded sleeve with an unpadded waist and chest strap. I can't fit very much in it and have to pack with a little care, but it sits between my shoulders like it grew there and I can hardly feel it. I don't really like riding with a rear rack or panniers, but for heavy stuff they're great. Heavy packs are the pits to ride with.

[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.