TL;DR: We enjoyed the apartment and would come again. Please take this as a recommendation :-)
We spent 5 days/nights in Barcelona with 4 people and can definitely recommend the apartment.
It is very central and yet still quiet that you can sleep without being woken up by the noise in each of the four bedrooms. Restaurants and other points of interest are mostly in walking distance or you can just take a bus or walk to a nearby metro station (5min walk).
If you prefer not to go out for food the kitchen leaves nothing to desire. Remember to bring some coffee capsules (nespresso) so that you can start the day with a cup of coffee :-)
Another pro: washing machine and dryer!
To sum it up: we were pretty happy with this flat and would come again.