I've been using the Tenba shootout medium. It fits in the airlines and more importantly, the ergonomics of the backpack harnesses fit very well on me. Not all backpack style bags will be easy on the back once 15 to 20 lbs. weight is added. I suppose that depends on each individual and their body frame shape. There are many backpacks that seemed good in the store, but once I added some weight they ended up the rear of my closet.
Negatives- Velcro. That stuff will ruin your gear if you do not cover it, turn it inside out, duck tape all of the loose hooks and loops off and vacuum and blow it off.
Thanks for the tips on the Lowe Pro and Bellingham. It's always nice to add to the collection.
Currently hiking with H3D, 50-110, 28, occasionally a 250, 150 and tri-pod total wt. 35 to 45 lb