Originally Posted by MichaelW
...Large bags encourage heavy loads which may not be suitable for your steering. I find that a few lbs of weight on the front does no harm.
+1 on this advice - unless your fork has a rack to support the bag, having lots of weight hanging on the handlebars is a sure-fire handling nightmare (I speak from experience on this one). I've gotten to the point where I prefer not to even have a bag on the bars at all! The added wind resistance of the bag itself makes the fork turn more easily in cross winds when I've no hands on the bars (about 50% of my ride). Maybe my experience is unusual?