Back in the day, I used to rock the panniers on a rack, but then I had to lug them around all day, which was not enjoyable. They also made the bike unwieldy when I had to stop at lights or in general.

I swtiched to a regular backback some time in the early 90's, and that was ideal for my 40km round-trip commute. I carried clothes, textbooks, Coke, etc. I liked the pack because it kept my back warm in the winter, and I didn't sweat like a pig to begin with.

Years later, I switched to a messenger-style bag, as I didn't need to carry as much stuff and my commute was much shorter. It was handy, but didn't keep my back warm and I couldn't carry as much stuff comfortably.

I recently returned to using a backpack, and it's perfect for carrying my lunch, lock, lights, plus spare clothing in case the weather turns sour. It's heavy, but very comfortable and pretty much bulletproof. I like how it fits close to my body and moves with me [waist and sternum straps are good]. I have to dodge and weave in traffic, and I appreciate the fact that my bike is light and manoueverable. When I bail, it's another bit of protection between me and the ground/car/bus/tree.

To the OP: no matter what you get, you will eventually get used to it. Try some stuff out. If you don't like it, sell it or just go and buy something else. Just make sure that whatever you use is the right tool for the job. Panniers should be of good quality with an appropriate rack system. Messenger bags should have a cross-strap of some sort to prevent load shifting/rotation. A backpack should have a narrow profile and should not interfere with your helmet when you're in a standard riding position. In this, as in everything else, you get what you pay for. Good luck!