I also own a Red Oxx Sky Train with Tom Bihn's Ultimate Strap, and it's been great! I can comfortably fit:

- A suit
- clothing for three days
- a laptop
- toiletries
- shoes + shoe trees

I did try Eagle Creek 15" packing folders inside the bag, but found that while I could use space very efficiently, I developed some pretty awful folds in the clothes, and now have been a bundle packing convert.