I spent 4 nights in a single room at Jon's Kaixo Hostel and I'm glad I decided to book there.
First of all, the location is smack down in the heart of the old town of San Sebastian. About a 15-minute walk from the bus/train station and only 5-6 minutes from the beach. Pretty much everything you'll want to see and do is within easy walking distance.
Given that it was a single room, it was more spacious than I expected. The bed was comfortable and the wifi worked well. I was afraid that I would have problem sleeping, what with being in the middle of the action, but the soundproofing of the window was surprising. That and the fact that the two bars downstairs close at about 11:00pm meant that I always got good night's sleep.
There is a kitchen for those who wish to self-cater and two bathrooms. There is a maximum of 6 guests (two single rooms and two double rooms), so there are seldom bathroom issues to deal with.
Jon is an awesome host. Communication was easy before I arrived and he was very responsive during my stay. He gives you lots of advice regarding tapas bars and restaurants and things to see and do when you check in, and he's quick to reply on WhatsApp if you have an questions during your stay. His suggestions really made a difference and my stay in San Sebastian was even better than I expected because of him.
Highly recommended!