Jamie, I just bought PacSafe bags for myself and my daughter for our upcoming trip to Rome. I don't feel that they are that much heavier than other bags, and I think they are very well designed.
I've used an Eagle Creek guide bag for the last 5 trips to Italy, and never had a problem, but then, we weren't just in Rome - home of pickpockets galore!
I really like that the zippers lock, more so than the wire mesh for protection from cuts.
Making your own "security" lock as some others mentioned on this board sounds harder to me, what if you loose that locking piece, or more so...they are harder to get open for you personally!
Most important in traveling is being aware of what is happening around you. No matter what, keep one hand on your bag at all times..protect that opening. Wear a money belt with credit cards/passport/cash. Only keep what you need for one day in your bag.