This thread and other similar ones on this board were very helpful in planning my trip, so I wanted to add a data point to help others. I arrived in BA this morning with US$1000 for a four day trip with the plan being to exchange US$500 and then paying for what I can in US Dollars with the remaining US$500.
I'm staying at the Park Hyatt and prior to setting out this afternoon, I stopped by the concierge to ask if they could recommend a cambio. They stated that they could only recommend bank branches as that is the only legal exchange, so I thanked them and headed down to Calle Florida. I wandered around for awhile and eventually felt comfortable enough to talk to a gentleman posted up by the Ateneo just south of Viamonte. We agreed on a price of 13,00 (today's rate is 13,38) and then I was lead into a building (with a security guard inside) and into a travel agent's. The process was quick and seemed more legit than some of the horror stories on TripAdvisor. They were also doing some booming business and it looked like a couple of the guys all brought clients back to this place.
Anyway, I never felt uncomfortable for a single moment and I'm glad I chose this method instead of Xoom.