In the cold weather, I prefer a backpack because it's easier to get my stuff inside quickly. In hot weather, I prefer panniers. I rode a backpack in 86 degree weather yesterday and forgot how bad it could be.

Anyway, this is the backpack I use:

http://www.brightideasusa.biz/backpack.html

It's a bit small if you're carrying shoes and lunch and clothes, but it works. I wanted something I wouldn't constantly be messing with reflective strips on.