I'm looking for a new bag to carry all my stuff in, that would go on my rear rack. I started with a backpack, and it just made me too sweaty for when I got into work. I definantly am happier carrying a bag on my rear rack, but I've never been really thrilled with the panniers I currently use. I'm thinking a bag instead of panniers hanging down the sides would be better... but I'm not sure. So, instead of asking what all your opinions are (since everyone's situation and tastes are different), I'm asking, "What do you use to carry you stuff in, and why does it work for you?"
As always, pictures help.