I've looked at this bag a few times at my local REI and seen it "in use" in the airport. Definitely take a look at it in person and bring some test packing materials.

To me, the useable space seems a lot smaller than the stated volume. The caveat is that I haven't actually test packed the Osprey and appearances can be deceiving (though as someone with more than a handful of carry-ons and often one bags, I'd say my luggage spatial sense is pretty decent). The bag's back lining is tented tautly over the pole and the top pocket also attaches at the sides to create some weird shapes to pack around. The kick plate between the wheels is rounded. It doesn't seem like it would fit shirt folders or some larger packing cubes, or even a dense bundle very well. People who roll might have better luck.

Then again, packing less makes the weight lighter but then I'd just bring a 18-20" instead.

There's a North Face 19" wheeled duffel and Eagle Creek Load Warrior 22" Wheeled Duffel that weigh a bit more and they both have an outer pocket.