This is a perfect spot for a Bilbao visit, you're only a 10-15 min walk from the Guggenheim to the North and the old city to the South East.
We were there in August and it was 25ºC, the apartment was a little warm at night but not too bad for a few days. I'm very glad it wasn't 30ºC+though as there is no breeze through the windows.
The facilities were very good in the flat though and I would definitely recommend.