Can't do anything other than give a very strong recommendation!
Location is great - easy walk into the main part of the city, a very pleasant walk along the river to the Guggenheim museum, and right by the old town. Also right by a small supermarket and also a cafe, so a cafe con leche first thing is just a few seconds away! Although the front door is by a busy road, the bedrooms are to the rear and so have zero road noise.
Aintzane consistently responded to communications within an hour or so and was always very clear and helpful. Very welcoming on arrival, and provided us with an overview of all the local amenities, main locations, where to find good places to eat etc. Very friendly too - great hospitality. Also made it clear that she was very happy to help should we have wanted more during our stay (which we didn't). We were on an early morning flight into Bilbao, so although check-in time is 16:00, it was fine to leave our bags on arrival before enjoying Bilbao. In fact Aintzane had everything clean and ready for us when we arrived around 10.45am!
The apartment is very spacious, comfortable, and had more facilities than we could possibly have used in the short time we stayed there (we stayed just 2 nights for short city break).
Bilbao is a great place by the way - very clean with plenty of places of interest, particularly if you enjoy art (as my wife does). Should we be staying in Bilbao again this would most certainly be where we'd stay.