Any frequent flyers here?
My wife and I fly a bit domestically in the US, so I'm not sure our experiences are a direct parallel to yours. But I agree with you that a backpack is a great solution for traveling with photo gear.
My wife has the Lowepro 300 AW vertex. It is maximum size for carry on when bulging with camera gear, but it does work. It will not fit in the overhead of some of the small commuter aircraft, but in those cases the airline simply stows it for you, so there is no baggage charge (again, US domestic policy).
I travel and use this Promaster:
I like it, and it always qualifies for carry on, and stows in overhead bins, no matter how full. I can get a 15 inch laptop in a dedicated compartment as well. For me, this is a good compromise, because I use it daily as a camera bag, even when a backpack isn't needed.