Ibon is renting out 3 rooms in this apartment. I had the smallest one. It is really small (as described) and in return you pay much less than for most rooms in San Sebastian (which is generally not a cheap place to stay). I was very happy with the room (and its price). Clearly, if you need a chair and a desk in your room, you should try to get one of the other two rooms. The room has a window to a relatively quiet street. With the window closed, you hear essentially nothing from the street. I also didn't experience much noise from the other two guests, so I can't confirm the negative reviews from others. I would guess that in general Spain is louder than maybe Germany or Danmark, so if you're super-sensitive to noise, then maybe this is not your county.
The apartment has two bathrooms shared by the three guests (and very occasionally Ibon), so there's never a queue. Everything is very clean. I didn't use the kitchen, but it appears well-equipped.
Ibon is a great host. He even picked me up from the train station (which was very nice on that rainy day). As previous reviews say, he does not speak much else than Spanish, but Google translate is good enough these days that we communicated without problems via Airbnb. When he showed me the apartment, I think I also understood everything.
The neighborhood is very nice with lots of small bars and restaurant, which see more locals than the ones in the old town. Nevertheless, the old town is only a short walk away (say 15 minutes, half of this along the beach).
I really liked staying in this room. As said, it's small, but comes at a great price, so if you plan to rather enjoy the city than sitting in your room, this is a great deal!