Bought it in a 45l as a gym bag and light travel bag. The shoulder straps are great although the addition of a chest strap would have been killer. When worn as a backpack, a half filled bag sits kind of weird on the back. Definitely comfortable on a short (15-30 min) approach to the climbing/camping area or a quick jaunt through the airport, but not great for an extended walk. Perfect for light travel, tons of features for the price and absolutely fantastic construction. Only had it for a little while but already put it through the ringer on my daily gym bike commute and it has held up fantastically both strapped to my front rack and strapped to my back. Will recommend!
So I've only had this bag for a little bit but I'm totally in love with it. I've used it on 2 trips plus everyday use and it really comes in handy. I bought the 45 and it works great as an additional carry-on / under the seat bag. Pretty much exactly what you'd expect from Gregory.