Overall, we had a great stay and you truly can't beat the location. 10/10 for exploring San Sebastien, and less than a two-minute walk to all the pinxtos bars (also close to shopping, beaches, etc.) If location is your priority, stay here.
That said, there are a few areas in which this flat could be improved. First off, the floors felt very dirty - we are a no-shoes indoors family and are feet were constantly dirty, covered in dust, crumbs, etc., from walking around the flat barefoot. The floors should be more thoroughly vacuumed and cleaned in between guests, and/or slippers should be provided.
The bathroom was also quite lacking in a practical sense; no hooks to hang our towels (I'd recommend adding hooks behind all doors, in both the bathroom and bedrooms), no shelf in the shower to put our toiletries — would also be a nice touch of the host left some body wash and shampoo in the bathroom.
There was (and probably will be for quite some time, judging by the looks of it) awful construction going on directly outside of the flat during our entire stay. Constant jackhammering made for an unpleasant experience so we didn't wind up spending much time inside the flat during the day. This is something the host should have warned us about.
That said, Maria was very lovely and easy to reach, the flat does have a lift which is quite convenient, and again, the location is amazing.