I have a Blackhawk 3 day "tactical" pack but it is the last thing I would want to carry on foot for long. It is perfect for car or airline travel though.
What someone said about lots of pockets is dead on. For luggage the Blackhawk is perfect. I also use it for carrying large amounts of grocerys on my motorbike.
It is tough enough to handle heavy items and the drag handle is very good.

I have a a Trekk pack which is ideal for backpacking but horrible as a day pack or luggage because it has just 2 huge pockets that are a pain top open. You can wear it all day though and stuff does not shift. It is also very light even with very padded straps and belt and aluminium frame.