I prefer to ride without vs with, but I do use an AlpineStar Tech Aero I picked up from GP Bikes fairly often. If it is fully loaded I do notice the weight when leaning in turns (tend to try to be more upright), however with only the minimals in it (water bottle, cloth and solution for cleaning visor/paintwork, deodorant, etc) I hardly notice it at all. I also use an AlpineStar Biotic back protector, so the hope is if I do come off the bike, it will help protect me even with the pack in place.
My only big gripe is if a bag is small enough for riding, it never seems to hold quite enough. I can fit what I need into mine, but would like it to be just a touch larger 9 times out of 10.