I have just spent two months very happily in "M49 - Serenity Beach - Design Flat with Sea View".
1. Apartment: The apartment is small, as, I suppose, are most apartments in Barceloneta, but very nicely designed and laid out. It would be suitable for a single person or a couple. It was quiet and peaceful, and I was very comfortable there. I have suggested that there is space for a small desk, but I was able to set up my work environment on the kitchen island.
In my opinion, the apartment's location is ideal. It is roughly equidistant from the Metro and the beach (approximately six minutes on foot for me), and tens of restaurants are within ten seconds to five minutes walking distance.
2. Neighborhood: Barceloneta is wonderful. Everything is on a small scale, unlike the Eixample, and the necessities of life, such as food, hardware, bakers, and barbers are, literally, just around the corner. The quiet and lovely Placa de la Barceloneta, just up the street, is a fine place to sit and meditate.
3. Landlord: Jirka, and his manager Alex, are, for me, ideal. They responded quickly to the only problem I had (a failing clothes dryer, which is now replaced), were helpful when appropriate and unobtrusive otherwise.
I give five stars to the whole experience. In fact, I hope to spend three months next year.