This is the kind of flat that AirBnb was founded for, and the one you'll be telling people about when trying to convert them. The design is absolutely beautiful, and we understand from Ibana that it was completely overhauled when they moved in. You can see this in every detail of the place - no space wasted, everything considered and finished stylishly. It really is a pleasure to be inside it.
The location? Right in the heart of the old town, which was buzzing right through our stay. However the flat is high enough above the street not to be too noisy. It is also close enough to the centre to walk to anywhere of interest in the city, including a nice walk along the river to the Guggenheim. There is a supermarket opposite, and of course many tapas bars etc a stone's throw away.
Ibana was an attentive and helpful host who gave us some excellent recommendations during our stay.
All in all, I can't recommend this place enough. It is a little more expensive than the average accommodation in Bilbao, but you most certainly get what you pay for.