I've been looking at luggage options for a while and just wanted something that I could carry stuff for a couple day trip. Ended up getting the Bags Connection
Rear Bag from Twisted Throttle
this week. Mounted it up and did a test ride with it today.
Installs pretty quickly and securely using the supplied straps at 4 points.
The specs say it has an aluminum frame, can't really tell, but it holds it shape really well even when it's empty (shown here).
I haven't verified it, but it's advertised being waterproof and comes with a waterproof inner bag too, for extra protection. When the top cover is closed the main compartment zippers are covered, and the side pocket has waterproof zippers, so it looks like it should be for the most part.
Expands if you need more space.
I rode with it today for about an hour and it stayed very secure on the bike. It came with some paint protection film, but I just used some pieces of foam pipe insulation where the straps hit the rear cowl (Worked great and stayed in place). It does encroach a lil onto the driver seat so I could feel it against my back while I was riding. Not really uncomfortable, but I could feel it there.
All-in-all seems to be a very well made bag with some nice features for a couple day trip on any bike with a passenger seat. It was a little more pricey than some of the other bags I was considering ($175), but I'm happy with it so far.