Today I really tested my new pannier bags, as my food supplies were getting low and therefore I had to do a somewhat demanding shopping expedition.
Everything was packed up properly in the bike, and its handling was still quite all right (of course, you could feel the inertia of the extra load – I tried to avoid going faster than 12-13 mph for that reason!).
Obviously the weight is something to be considered, especially when living on a hill! 🙂