Merce's apartment is perfectly located! My son and I were able to walk everywhere! And I mean everywhere! We logged about 10 -12 miles each day we were there, walking to the various sights. We walked to Casa Batllo, Casa Mila, Las Ramblas, the Gothic Quarters (one of our favorite places!), restaurants - so many! (and our host gave us excellent suggestions for restaurants), supermarkets, etc. Everything is walkable from the apartment, even if your not as avid walkers as we are. We went to Montjuic by metro. The metro was only a few blocks from the apartment.
I would recommend Merce's apartment. It is not luxurious, but it is comfortable, affordable and in a convenient location to exploring Barcelona.