I've been using the same bag as my personal item for nearly three years and it may be time to change. (I'm currently using a Tom Bihn Pilot). I'd like to hear what others are using especially if they meet my basic needs:
1) No bigger than 40 cm x 30 cm x 15 cm. (15.75" x 11.8" x 5.9")
2) MUST Have a trolley strap to fit over the telescoping handles of a wheelie bag.
3) Not too heavy (this is subjective).
4) Zippers are somewhat lockable.
There is no need to mention any Rick Steves bags as I am familiar with them.