This apartment is really cool in a super cool town. If you are thinking of going then just GO! We caught the bus from Bilbao for 12.50 euros and the bus station in San Sebastian is right in the centre. Getting the keys from the Rentals Collection Office was extremely easy and the staff so warm and helpful.
We stayed for 3 nights. There is a large supermarket about 2 minutes walk across the street. It is easy walking distance to the old town and the beach. The town is full of interesting places to visit and we particularly enjoyed walking the coast, it's stunning. We took the funicular to the west high point and the views are spectacular (the fun park is a bit run down but just keep your back to it, under 10's would like it).
The whole apartment is really lovely and the terrace has the sun from about 11am to the evening. The bed is super comfortable and the double blinds make it completely dark. It's also a quiet apartment.
As I said, just go!