My wife and I just returned from an 8 day trip to London and Paris. We each took just two items, with our main bags the Rick Steves Convertible Carry-on Backpack. We bought these bags for a trip to Scotland back in 2002!
They've done a lot of traveling since then. We chose these bags for our latest trip because we knew we didn't want to take any checked luggage and wanted something that would be easy to carry on subways and on the Eurostar. We had plenty of room and fit in the overhead compartment even when packed for a trip this length.
They are tough as nails and have held up extremely well. They are very comfortable to wear as a backpack. We are in our '60's and I have to help Deb get her backpack on her back, but once there she's a real trooper!
I heartily recommend these bags. I also heartily recommend the RS philosophy of traveling light, especially if you plan to be moving about and taking public transportation.