I needed a tailpack for occasional commuting (and fits a laptop!)and weekends away. Something waterproof, decent quality and which would fit onto the pillion seat. I went to the excellent Bikebitz in Yately (https://bykebitz.co.uk/
) as they've got loads of luggage which you can poke around and trial fit. Big thumbs up to them.
I settled on the Kriega US30, it's 50% bigger than the US20 and only about 20% more expensive.
Turns out it's perfect for weekends away. I got 2 pairs of shoes, a load of clothes and other stuff in. The only thing is that it's a roll top type bag so you need to pack carefully and make sure your stuff's well folded or rolled so the bag keeps a decent shape.
Once you get the 4 straps torqued down it's very stable and secure on the bike. The rear subframe of the XSR also provides good mounting points for the straps. It's definitely a quality bit of kit.
The only shame is that you can't keep the straps which attach onto the bike in place without the bag as there's not enough space to tuck them underneath the seat. I guess the XSR isn't meant to be a practical bike: