5 stars, with a caveat.
First, this place is as beautiful and amazing as it looks in the pictures. The views of Rio are stunning and you feel on top of the world. It really will fulfill your daydreams/imaginings of staying in Rio with a view.
The apartment is very clean, new, and comfortable. Everything you need is there and there is a ton of natural light. You do have to walk up a hill, but, if you are not feeling it, cheap motos can take you almost all the way to the apartment.
The caveat: The apartment is in a favela; however, Babilonia is notoriously safe and you will see the heavy police presence. There are several hostels within the favela. Going to and from the apartment to town or the beach is safe and is well-lit 24/7. We came in day and night without issue. That being said, I would not suggest exploring all corners of the favela or hiking the tourist trails above the favela, unless you are comfortable being interrogated by armed men with unholstered glocks. It may have just been the wrong time to be there but my girlfriend and I were walking up a nearby viewpoint midday when we had such an occurrence. We found ourselves accidently intruding on a few lookout men and were approached and questioned (in Portuguese, which we do not speak) about our business. They did not harm us and ultimately encouraged us to go up the trail, but it really takes the piss out of your will to hike when a man with an unholstered gun is telling you to walk into a jungly area. So, just always remember you are a visitor there and be very respectful. Also, do not take pictures of the houses in the favela!
Really, we enjoyed our stay and the uniqueness of the location. I still think it merits a five star rating but it might not be for everyone. We were very comfortable and felt very safe, aside from our hike to the viewpoint.