For a couple wanting to explore downtown Bilbao and the Guggenheim the location is ideal. The apartment was fashionable, spotless, and well equipped. I would absolutely stay again.
The only caution I would make is the apartment is the 2nd floor (1 in local parlance) and on a busy street. In warmer weather it is a necessity to sleep with windows open, and in this case the noise from traffic and people on the street could keep lighter sleepers awake. Honestly I imagine the same might be said for any AirB&B so centrally located.