The only negative experience I had was the staff not being able to manage, understand and totally ignoring the request of sending me a water kettle to my room to make tea with. I had an upset stomach the last night of our stay and that would have helped.
We visited Hotel Miro in mid July 2015. The lady on reception was very nice and supportive. When the airlines lost my luggage she made sure it was sent straight to the our room as soon as it arrived. She was very welcoming and answered our detailed questions about Bilbao.
The hotel is clean and well kept, it feels like it has been around for a while but that didn't bother us at all. The Ac, hot water and everything else worked great in our room. We got a room that had a bench and you could see the Guggenheim from the window, I would recommend a room like that. Only make sure its higher than the seventh floor so the building next to the hotel would probably not be in your view.
Also at the bar area juices, water, tea & coffee were complementary till about 10:30pm and that was nice from them.