San Sebastian is a lovely city and the location of the apartment is perfect for walking to all of the popular spots. It's also a two minute walk to the bus stop that picks up/drops off for the E21 airport bus. Super convenient and affordable ride to the San Sebastian (EAS) airport (it was 2.75 euros).
The pictures on Airbnb are accurate. The apartment is recently renovated and it was very clean. The kitchen is well stocked with at least eight place settings, utensils, and other kitchen tools needed for cooking. The WIFI was excellent and allowed me to join work video conference calls. Note that the wall to the second bedroom is glass - it is shown in the picture, but may be easy to miss.
There is construction going on in the building, and during my stay, it was very loud at times. It looked like some structural work was happening, and it's possible that by the time you stay it will be complete. Note that the workers were only there during the day, so it was only a problem when I needed to stay in to do some work. Note that without the construction, it was very quiet. The apartment windows face the interior courtyard, so there was no traffic/street noise at night.
I did suggest that the hosts add a mattress topper to the main bedroom bed, as I found the mattress to be uncomfortable.
Overall, I thought the apartment was really nice and would recommend.