We stayed three nights at Marta's: the flat itself is one of the highlights of our stay in Bilbao - as I said it's one of the city's three best landmarks.
Communication with Marta was top class since day one, and she made sure we had all the info before arriving. She met us near the entrance of the underground and secure car park (essential if travelling on the road), showed the flat and we didn't hear of her until the sad time we had to leave - not that we needed anything in the meantime.
There was no reason of concern and no complaint about the place: it's very stylish, perfect for a couple, and near to everywhere. The train station is only five minutes' walk away and most places of interest can be reached on foot anyway.
We couldn't recommend Marta's flat more.