We stayed with my friend at Pilar's place for three nights and we are overall very happy with our stay.
Pilar's apartment is very big, cosy and clean.
Pilar is an excellent host, very friendly and helpful. Despite the language barrier, she helped us with useful tips on things to do (where to eat, what to visit, how to move around). Also, she was very hospitable and made us feel like home.
The location of the apartment is perfect, right in the heart of Casco Viejo with lots of restaurants, bars, cafes within walking distance.
I would definitely stay at Pilar's place when I visit Bilbao again.