JJ's flat is superbly located just a few minutes walk from Calatrava Bridge near the base of the funicular. Everything is within easy reach - the Old Town is about 15 minutes stroll in one direction, the Guggenheim about 10 minutes in the other. There is a bakery nearby, as are three supermarkets and a great local bar downstairs.
The flat is spotlessly clean and really spacious - the photos don't do the living area justice. There was everything we needed for a weekend in Bilbao, and JJ was extremely helpful when our inbound flight was delayed. Overall I would thoroughly recommend.