I couldn't be happier with my stay in Alex&Anastasia's studio in Copacabana.
The place looks exactly as in the pictures; it's very modern and well maintained. It's located 2 minutes away from the beach, so you could go jogging in Copacabana - I got to do it every morning during my stay there. Moreover, the studio is in a building which has a 24hrs doorman, so it's super safe.
Finally, the hosts were amazing - Alex and Anastasia went out of their way to make my stay very enjoyable. Thus, if you want to have a great experience in Rio/Copacabana, then I would totally recommend this studio.