I really enjoyed traveling with the Saddleback Leather Medium Waterbag along with my Saddleback Leather Medium Thin Briefcase. I didn’t have any problems except for one small plane where the waterbag didn’t fit in the overhead compartment and had to be checked. I could easily carry a weeks worth of clothes.
If I had to express one concern it would be the weight. Trying packing the waterbag from A concourse to D concourse at DFW, I don’t suggest it unless you are hardcore (which I am, I don’t mind at all). It can get tiring.
What would really be nice is if the medium and large sizes came with the same sort of backpack conversions that the briefcases come with (update: it does).