First the positives:
the apartment is very clean,
the hosts were very flexible about my extremely late check-in,
they were friendly,
and communication was easy.
The apartment is also in close proximity to a lot of the attractions in the centro historico.
The bed was reasonably comfortable,
and the wifi worked.
The kitchen is simple but equipped with the basics and they left some beers for me (which i didn't drink, but appreciate the touch).
The negatives are that the immediate vicinity of the apartment is not nice. For whatever reason there is no lighting at all on that city block, and the street and sidewalks are torn up (nov 2017) as they work on some city project. Additionally, this is in the not-so-great section of the centro historico, although you're only a couple blocks from the main streets.
Finally, the noise was absolutely unbearable. Part of the reason was a monthly festival that goes through two nights and involves fireworks at all hours of the night. Avoid any housing in the center on the 27th and 28th of each month. But the building itself was also noisy.
I feel bad complaining about these things, considering that the hosts are doing their best and seem to be very nice and conscientious people. The negatives are really beyond their control. If you are comfortable with divey streets and are a heavy sleeper, then this is a fine and very affordable option.