Overall the apartment is clean and well prepared, it has the most important amenities. it is located around 15min by foot from San Mames stadium, which is pretty convenient if you are a walking type, as you can reach most places in the center by foot (e.g. Guggenheim is around 30min walk).
The company managing the apartment are also very responsive and professional. The apartment, while not the most spacious, can easily fit in 4 people, has a good kitchen and a small living room.
At the same time, several things have to be taken into account. First of all, the apartment is right above a bar, and right outside the building there are train tracks, so while it is not the loudest place that I've stayed in, there are certain disturbances regarding the noise. The walls in the building are also not the thickest, so expect that you might be hearing your neighbours' activities too.
Also, as it is a newly arranged apartment, there are still some things to be worked out - e.g. boiler broke down twice during my 4 day stay, which meant that twice the apartment was almost freezing and had no hot water. Luckily, the owners reacted promptly and by the time I left it was at least temporarily fixed.
Overall, it is a pretty ok place to stay if you dont expect to be blown away and just need a clean, normal apartment to stay.