I posted a few days ago that I had purchased this backpack,
http://www.hmhd.com/webstore/viewite...p?sku=94751-08 (shameless plug for the shop I ordered from also). I finally got my sissy bar and luggage rack installed last night, and used the backpack for the first time today. Have to say I am thoroughly impressed. This thing was definitely designed by someone who rides, and understands the needs of riders. It can be secured or removed in less than 30 seconds, and due to the 3 mounting straps it's secure and definitely not going anywhere. There is a large pocket on the back of the bag that secures the shoulder straps so they don't flop around. Sure did take a load off my back, literally, by getting my laptop off of my back for the commute to and from work.
I mentioned in my other post, how roomy the bag is. Would definitely make a good overnight bag, and if you pack conservatively you could probably stretch it out to a 2 nighter. Also comes with a rain cover, just in case, and good pockets on the side for keeping stuff handy such as a bottle of water.
I realize $150 for a backpack is steep, but if you are in the market for something like it, I highly recommend this one.