I highly recommend staying with Ainara and Rober. My husband and I stayed with them for 5 nights and it was even better than expected!
We had our own big bedroom with a desk, tv and plenty of space to store our things, our own bathroom (also big with plenty of counter space, great shower) and the apartment is in a great location within Casco Viejo. It's on a street with lots of restaurants, close to the train station, and about 20 mins to the Guggenheim.
The best part was Ainara and Rober's warm hospitality. When we arrived, Rober showed us around the apartment, including the kitchen if we wanted to cook. They also gave us a bottle of white wine, as well as a cake along with breakfast each morning. We had the opportunity to get to talk with them, but we also had privacy if we wanted it - a great balance.
The apartment was great, but the hosts are even better. Don't hesitate to book!