I thoroughly recommend staying at Julen's apartment. I stayed there for a single night with my wife and teenage son prior to flying out of Bilbao, but would have been happy to have stayed longer. The space was very well equipped and clean, and the bed (and pull out sofa bed) were also very comfortable.
Contact with Julen was easy. Prior to arrival he sent through some location details. The apartment is attached to his house and he met us upon arrival, providing some really useful advice on the local area - he and his wife were really nice and were happy for us to keep bags in the apartment for the day after our stay, as we weren't flying until later in the day, which was much appreciated.
The local area is residential and quiet and felt very safe. We decided to check out a rotisserie chicken restaurant, which was a 5 minute walk away (Batzoki) - definitely not a regular tourist spot, and full of locals - a great atmosphere and we loved trying to communicate in our extremely limited Spanish!. Bilbao itself is about a 20 minute drive away, and easy to get to the Guggenheim etc.
Thank you for your hospitality Julen, it is much appreciated, and we hope to return to your city again.