I tried the Arkel handlebar bags (small and large) and found the small too small to be useful, and pretty damn heavy, and the large big enough and ridiculously heavy. Who needs a bar bag that weighs half as much empty as the total load you'd like to be carrying? To me, big enough means big enough to carry wallet, cellphone, camera, a little bag of medicine, enough food for 100km, and have enough room left so that I can easily stuff into it the cold weather gear that I was wearing in the morning and that I don't want to stop to put it in my saddlebag or panniers.
LonePeak Altra H-100 has a bit more carrying capacity and weighs half as much. I've ridden with it for thousands of miles with nary a problem. It isn't supposed to be waterproof, but its never gotten wet inside, even in downpours. It mounts easily and securely, yet its easy to pop off the mount and carry it into a store. For another thread on this forum on the same topic, see
http://www.bikeforums.net/showthread.php?t=405443
Don't get me wrong, I love Arkel stuff and have their front and rear panniers plus tailrider. But the bar bags are just unnecessarily heavy.