We stayed at Denise's apartment for three weeks. Our experience was really good. The apartment is super comfortable, spacious and quiet and has all the amenities you might need inside. It's also very well communicated with the main sights and interesting points in the city (including the University of the Basque country which is just 5 blocks away).
Wi-fi functioned well and allowed us to work normally without any inconveniences.
I recommend Denise's place if you want to visit Donostia :)