I don't really like riding with my pannier, but, I do it when doing errands. I couldn't do a regular grocery shopping, even with 2, at least only once a week, like I do. Too much stuff.
I totally agree with the backpack/messenger thing. I firmly believe my friend broke his hip, not just because he slid on black ice, but because he was riding with his laptop on his back.This is someone with good handling skills. I tried commuting with a stuffed messenger bag and it was bigger than me! I ended up taking the bladder out of my Camelback and using that as a backpack. I hadn't done that for a few years, when I needed to bring some stuff to change into after a group ride, I was riding to (7 miles). It was painful, and the top of the pack kept bumping into the back of my helmet.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport