I used an REI-65 Flash backpack for a 2.5 month trip in Asia/England this past summer. It was crammed under seats and in overhead "bins" (wire cages), sat on by multiple people (at once), stuffed in trunks, and thrown on the top of buses (often under heaps of other goods, including a couple of car tires and a motorcycle for one 25-hour journey). It held up quite well, with no new holes or rips, just permanent stains. I also used it for a couple of "real" backpacking trips during the summer (a 3-day and a 6-day trip). At 3 lbs, it isn't super light, but it is very durable and adequately handled some pretty severe abuse.
To help protect the straps and hip belt, I'd adjust them so they went backwards over the top and around the front, where I tightened them and tied them together. In the past, I've used a backpack that had 'zip-away' straps, but the bag wasn't really good enough to do a "real backpack" with. (Bags have come a long way since then, however.)
The Flash-65 is too big to contemplate checking it in, but I can't check in bags like I used to anyway, due to the banned lists of goods I carry (fire starters, knives, stoves, and so on). I don't know how it compares to the Flash-65, but the large version of the Flash-45 has a capacity of 50 liters, the size you are aiming for, and weighs 2 lbs, 2 ounces. Both are pretty cheap at the REI outlet right now, if money is a factor ($94 and $64).
One suggestion: before buying, make sure your stuff will fit in a 50-l pack, and if you don't have "full suspension" (or whatever it is called), make sure the weight of your gear will work with it. I have a Jam 50 but didn't use it because (1) all my stuff would not have fit in it, (2) my stuff would have weighed too much for the pack to be comfortable, and (3) I've never actually used the pack... :-)
EDIT" The Flash-65 is too big to contemplate NOT checking in...