Age old question with a twist. After much thought, I've decided that panniers just aren't my thing at this point in life. That being said, I do need a way to carry my stuff while on the bike.
I've been interested in mess bags for a while, but have as yet to pull the trigger on one. Mainly because I think that since a mess bag is designed to rest on your back while riding (best done in a more aero position- a la road bike with drops), using one while riding in a more upright position on a hybrid, they wouldn't be as comfortable/practical. I could be wrong.