Rear panniers will just make your bike feel heavier and less responsive. Front panniers will initially feel like you can't steer the bike anymore because the extra weight that you have to move when you try to turn the handlebars. But with a few hours on a bike with panniers, you get used to them and can handle a bike well. But you do not want to try to go around fast corners with heavily loaded panniers, just like a semi-truck driver has to slow down for sharp turns.