This apartment is a gem - and we would highly recommend it to others.
The location is unbelievably great. Nicely situated in the old city - it is just a street over from the heart of great nightlife (Spanish nightlife - which is a much more genteel and charming notion of nightlife than the phrase connotes in the US). The square it looks out on is pleasantly bustling by day and into the evening - but quiets down at a reasonable hour such that we never found our sleep to be disturbed.
Architecturally the apartment is different than many European layouts. Rather than being a shoebox with the living space facing out and the bedroom tucked back into the center of the building with access to a lightwell - this apartment runs lengthwise along the face of the building and subsequently every room has windows (again - looking out to a picturesque square). It is (in our experience) unique and delightful.
The apartment is nicely renovated, maintained and appointed. Clean and modern. We definitely found ourselves feeling as if we could actually live here for an extended period of time - not because it is especially large but rather well laid out. It was extremely comfortable for two. Had we not been so charmed by Bilbao we would have liked to have spent more time actually enjoying the apartment itself.
There were many thoughtful touches. A selection of literature about Bilbao was available. As a welcome gift there was a bowl of fruit as well as some baked goods (bread and muffins). The fridge had some milk and yogurt and there was COFFEE. Which was great.
The ability to park a car was also very handy for us (the old part of the city is essentially without parking and on the weekends without cars - so the space is about a 15 minute walk away). Getting around Bilbao was easy without ever needing the car - but the north coast of Spain is worth exploring.
In the end I would say the words we always look for when reading these reviews - we would absolutely stay here again.