I really enjoyed my stay in this apartment. It is quite spacious and well located in the old section of Bilbao (Casco Viejo).
The condition of the apartment is excellent -- everything was perfectly maintained and really clean. There are 2 large bedrooms, each with access to a complete bathroom, so it is perfect for 2 couples to share (one of the bathrooms is enormous, while the other bathroom is "merely" spacious). It is easy to travel here from the train station (the intermodal station) -- you just take the tram a few stops or walk 30 minutes. The apartment is quite close to the Casco Viejo metro and train station so getting around the city by train or street car is very easy if you don't feel like walking -- but I recommend walking!
The host, Luis Manuel, was very nice and accommodating. The care and attention to detail he showed were wonderful and enhanced my stay at his apartment. He is a great host!
The one negative is street noise, which is obviously beyond the control of the host. Casco Viejo is a lively area and people express themselves in various ways, sometimes loudly and sometimes late at night. If you are a light sleeper you may want to try a different accommodation, though I suspect street noise is common at many Air BnB accommodations in Bilbao.
One strong recommendation is to see the Museo de Bellas Artes. It has a very good collection that is arranged alphabetically by themes. This is quite unusual and the quality of the experience it produces is very high. Other highlights of my stay was taking the funicular up to get a panoramic view of Bilbao, and also visiting the Vizcaya bridge -- though the latter does take quite a lot of time, so if you are in a rush it's not a good idea to go there. The Guggenheim... well, that is a big building that holds very little art. You have to see it, but the Museo de Bellas Artes is really much better for serious art lovers.