I have a bag very similar to that made by Topeak. With the Topeak, if you have one of their racks, it can be attached/detached with their "quick track" system, which I really like. It was surprisingly heavy the first time I lifted it, but I'm not too concerned with weight. Here's the link...
http://www.topeak.com/2007/products/...runkbagdxp.php