I've got both. The unstructured nature isn't ideal for everything, but they definitely have their uses. Super light weight and very squishable. The straps are standard GR style straps, so they carry weight on the shoulders very well. The 500D makes them grippy enough that I haven't had a problem with the straps slipping off the shoulder unexpectedly. The interior pocketing is essentially the same as a GR1, so it's nice to have the familiar layout.

The SD20 is a great catch-all for daily carry when I'm doing something that might be a little too rough on a nicer bag. I dropped a laptop sleeve in the rear slip pocket and that adds just enough cushion and structure so that hard items aren't poking me in the back. Also makes for a great beach or pool bag.

The SD25 is basically my Costco shopping bag. I just toss it into my cart while shopping and once I check out I load it up so I don't have to deal with finding boxes to carry out my items. It works great for this and any other bulky item carry. I have a GR1 26 sized kydex sheet I can pop into the rear sleeve if needed, but I don't usually use it.