I have been using the StreetWalker HardDrive since 2010:
* Fits exactly under the seat of a smaller jet airplanes (smaller than the Boeing 737)* Carries extremely well* Carries too well, so I have a tendency of putting more stuff and making it heavier.* Ugly backpack
There are sales on regular basis.
Artak Hambarian: While have not used this bag, one drawback is evident: you need to put it on the ground - read mud, water, steep slope, ice - to change a lens. The art of bag making still needs to say its word in combining comfort, performance, speed and versatility, in my opinion...
I have used this exact backup in the arctic and had to put it down in the ice, the mud (carefully) and many wet spots. No problem. I don't know what material they use, but nothing got wet.