Really nice unit in a good location. Clean and modern interior with new looking kitchen and appliances. Elevator saves you walk up the stairs. Super-fast 100+ Mbps Internet. Ample room in the wardrobe to stow your stuff. Great shower. Washing machine well maintained and very useful in keeping on top of laundry reducing need to pack. Overall it was a great stay.
The things that could be better probably relate more to culture/custom/language differences from the U.S. and weather.
As revealed in the listing there is no AC. I understand it is usually fairly temperate in Donostia, so normally this might not be a problem. During our stay though the daytime highs reached 89°F (32°C). There was a fan in the main bedroom but none in the secondary.
Related to that, apparently in this part of the world what is referred to in the listing as a "duvet sheet" is what an American would probably describe as a thin comforter (there are also supplementary blankets in the wardrobe). We had another like it on our Madrid leg this trip. And as on that leg too, the custom evidently in this part of Europe is to have no additional sheet underneath said comforter.
Now normally this might be fine but in the warm weather the choice was the comforter or nothing. I'm a pretty hot sleeper so I wake up soaked in sweat. Then when I remove the comforter I get chilled because of the sweat. If you have similar issues you may want to bring a silk traveler liner or something like it.
The appliances are like new and top notch (e.g., Bosch). However, Americans may not find them intuitive and while there are some manuals in drawers remarkably they did not include an English section. Google is your friend here. Put that speedy Internet to work and search the model and you should find a pdf in English like I did. Still, it would have been nice to have a binder with some basic "quick start" instructions, but I'm nit picking here.
Again, this is a clean, good sized unit in a very convenient location (pretty much most things are within a 10-15 min walk). I would stay here again when I return.