I really like my Eagle Creek Conor backpack. It is checkpoint friendly and has a middle pocket that I use for clothing. It's very lightweight and has good straps.

If I were to get a messenger or attache style, I would look at Waterfield Designs (sfbags.com) first.

If your issue with a bag is the strap, you could always replace with this:

http://www.amazon.com/dp/B000EEMA8M/

It is the same as the Tom Bihn Absolute strap that is very highly regarded.