The apartment is beautiful and suitable for two to three people. The kitchen is well equipped and there are plenty of grocery shops very close by. We enjoyed breakfast on the balcony, which could probably do with a slightly more stable table. The shower works fine. We didn't use the a/c which is only in the living room. It was enough to open the windows. However: sleeping with open windows was great temperaturewise, but very noisy from the street in the bigger bedroom. There is a useful washer and a dryer in the small bedroom so you can travel light!
The metro station Rocafort is a two minute walk away. When arriving from the airport you can take the R2 train to Sants using the cheap T10 ticket which you can also use for the metro in Barcelona, absolutely no need for the hola bcn card. From Sants it is a 10minute walk to the apartment. During the stay I would recommend renting bikes. We only did that at the end, unfortunately. It is a fun way to disover the city and easily gets you to some nicer beaches beyond Barceloneta. The metro was also useful and discovering on foot was lots of fun.