This is a great apartment to stay at when in Bilbao. The design of the apartment is very artistic and spacious. There is amazing food all around and you're right down the street from the Ria, which is very nice to see at night.
The apartment is right next to Casco Viejo that has wonderful tapas and a wonderful celebration for Semana Grande. One of the best parts of the location is that Marta provides you with a garage for your car, which is great if you want to do something outside of Bilbao (such as visiting San Sebastian). The apartment is about a 20-25 minute walk to the Guggenheim, but if you're willing to walk a bit or take some transportation, I would definitely recommend staying. Above all, Marta was a great host who was super communicative and available.