We had a very nice stay at Marta's place. They were very accommodating in letting us drop our bags off prior to check-in, and they allowed us to check in a little bit early as we had scheduled a tour that afternoon. Artur met us at the apartment and gave us a bunch of great recommendations. Communication in general was prompt and clear.
The apartment itself was clean and cozy, and the balcony overlooking Sagrada Familia was a lovely place to sit. Note that the elevator in the building is particularly small and can only really fit one person at a time if you have bags. We also had an instance or two where other people in the building didn't properly close the elevator doors so we had to take the stairs. It wasn't a big deal at all - the stairs were frankly easier than the elevator - but something to keep in mind!