Ok, so as promised, a quick picture with the US-20 on the seat. This is with my phone so not the greatest quality.
Unlike the OP in this thread, I use the buckles to attach to the bike (rather than the metal hooks), takes just a few seconds, and use the straps to tighten. When the bag is off, the buckles fold under the seat so you cannot see anything.
This morning (on this picture) I had my laptop, a sweater, waterproof pants, a cloth, some other bits and pieces - but it could fit a lot more as well. To be honest, before I installed my Fehling rear rack it was an easier fit, since you could slide the bag further back above the rear fender. So if you don't have a rear rack, or you have the OEM one with no grab handle, then it'll fit even better.
There's lots of info out there about the bag itself, so I'll just say I found it to be excellent quality and like it very much.[/align]