I highly recommend Eike's apartment! Location is really incredible, it's in the city center, but it's pretty quiet. In 500 meters you can find several bus stops, Diagonal and Passeig de Gràcia metro stations, where you can get almost anywhere, including the Airport (for 1€ with RENFE train ;). The apartment is clean and comfortable. We stayed there in December, it's warm enough, however tile floor is cold, it was not very comfortable for our kids, so keep it in mind. Also notice that elevator is pretty small and our child stroller barely fit in the elevator (60 cm wide). This can be a problem because the apartment is on 5th floor, however Barcelona is built up with identical houses, just get ready for this.
Anyway, we had a great vacation in Barcelona, come here, it's worth it!