I spent some five days in this tiny flat, and I feel to recommend it wholeheartedly! It is a studio, but for a solo traveller is simply ideal. The location is just perfect, just a short (like 10min) walking distance to the seafront, the central Aristotelous Street, the markets nearby, and a great number of restaurants and bars.
I found the flat really clean and I was greeted with a delicious cake. The bunk bed is comfortable (the stepladder not so much, but it's a detail really), the sofa, the kitchen, ... I really felt at home. The bathroom might be tiny, but the shower is really good. the only problem I had was the extremely tiny sink - too difficult to wash my face on it, so I had to resort to the kitchen sink.
In terms of accessibility, consider that it is on the first floor without a lift unfortunately. And of course, it's a bunk bed (but I believe the sofa can turn into a bed easily).
Overall, it really was a great stay and I do recommend it. Thessaloniki is wonderful and this studio is the perfect nest to start exploring the city!