I bought this for a week-long trip to Iceland because it looked like I could carry it on if I zipped off the 15L attached pack. I was able to carry it on without challenge, but on smaller aircraft had to gate check while on larger aircraft (767) had to turn it sideways to fit in the overhead bin. On the way home, with gifts, it was stuffed and heavy so I decided to check it once I got back in the states for the last domestic leg of my itinerary. I flew American Airlines domestically to the connecting US international airport, where I picked up IcelandAir. Im a tall female (68 inches) with a long torso so the mens version is fine for me and with the adjustability it was comfortable and easy to use. The full open zipper (rather than stuffing the thing from the top) made getting to my gear easy without always having to unpack then repack. The zip-off pack was convenient and also easy to get things into and out of without necessarily having to unpack and then repack to get a specific item out. It was a great day pack on the trip. The whole thing is low profile compared to other packs and especially book backpacks. I have the reddish colored version and checking it got it a little dirty, but spot cleaning was easy and it held up (no scratches, picks, or tears). I looked at the smaller version (40L) but Im glad I got the 55L instead with the judiciously selected winter gear I needed and saving a little room for gifts (I squashed an Icelandic sweater into it for the way home). I recommend this backpack. Its roomy (all things considered), and allows flexibility.