I do feel a difference when unbalanced, but only if it is fairly extreme in the back (say 20-30 lb difference) or in the front (I do feel a 3 lb difference). No reason (besides financial) to be afraid running two difference panniers; I do so fairly regularly around town, with no issues. I do agree, however, with the posters who like the big black hole. It has much more flexibility than pockets. YMMV