The apartment is located perfectly. Near the center of the city but on a quiet upper floor. it is clean and very very roomy (6 different bedrooms, two toilets etc.) and with a nice private terrace.
Having said that, there are three problems.
First, the apartment feels like an office-space for rent. It is functional to its core and feels like a space a corporate will hire. The sofas, for example, are not comfortable to sit on, but they can open to a bad. Functional.
Secondly, everything is based on a penalty system. You lose a set of keys? You will be fined 100 Euros. Make noise after 10pm? 800 Euros (seriously). Check-in after 8pm? Add 20 Euros. 10pm? Extra 30 Euros. After midnight? Well, that will cost you 50 Euros and its not that the apartment comes cheap; far from it.
Lastly, and most importantly, the mattresses are simply horrible. My mattress was so soft I nearly drowned in it. We had to replace the mattress with a different one from a different room, but the size of the bed frame was different so, as you might expect, the sleeping was horrible. Add to that that the pillows were extremely soft and useless which made us use the sofa's pillows which were also useless for that purpose, and you get a really terrible sleeping experience.
These can all be fixed with a little touch and consideration, but it seems that this apartment is a business and not someone's love, so you get a business like treatment which is a shame: it is really a beautiful place.
Unfortunately if we will visit again in Barcelona, we will not choose it again.
Sorry.