I had a Deuter ACT 50+10 for years and traveled with it all over BUT I carried a daypack jammed into the 50+10 and threw the whole assembly into a duffel at the airport and checked the pack.
I ditched the daypack and duffel to carry just one pack with me; the ACT 40+10. I use a foam insert for my camera gear (a Nikon DSLR and a couple lenses). This way I carry it all on the airplane with me. The 40+10 fits into overhead compartments when you do not extend the pack.
I strip out the camera gear insert when boarding and the pack looks like a daypack and I am carrying a handled foam box.
The 40+10 is that perfect crossover size, fit and comfortable - small enough to carry as a daypack, small enough to fit in overhead compartments, comfortable enough to carry in the tropics loaded with 30lbs (I rarely carry more than 18 pounds). I carry it on trains, buses, tuk-tuks and of course, my back.
The only thing I question is the long term durability of the stretch fabric. I try not to use the back pocket because of that. Oh, and that I had to locate a suitable silicon nylon rain cover. They should include a cover for a pack that works so well in the tropics.
Lastly, I am 50, 6' and 210 and this pack ( and the other Deuters I own) is a truly comfortable pack for all sizes. They figured out the suspension system that works for nearly any back length or body. My wife, who was using my 50+10 has now picked up one of these AND my son wanted his own new 50+10 (he will check his)! I have a lot of backpacks in my usage collection, from daypacks to my Bora 80 and Lowe SAR pack - and this pack wins, hands down for everything from a daytrip to overnight to, well, a three week trek in SE Asia. Expect to see it on the Camino Santiago in the near future!