Marbella is a lovely and charming neighborhood inside the city of Panamá. It is considered mainly a place with some high apartment buildings, offices and shops. It is a very short distance from the banking area, the Paitilla Hospital, Multicentro Mall, the World Trade Center, the Santuario Nacional Catholic church, many restaurants and other many important places where you can go to do anything you want.
The only negative thing about Marbella is the heavy traffic almost at all times, because it is one of the ramps for "Corredor Sur" (a highway with heavy traffic), and some important local streets. But overall, it is a nice place to be.
Regarding expats, Panamá is considering a census on foreigners residing in the country, including, of course, the city of Panamá. But there are many thousands of them, with many, many from the United States and Canada. Stating the quantity of expats in Marbella would be misleading, because many live around Marbella, in other neighborhoods as well, and Panamá is very small; a count of the residents only in Marbella would not give you an accurate response. And they all are very happy with the choice of taking the decision to live in Panama.