We've stayed for one month. The flat is exactly as you see in the pictures and clean. It's very central: more or less in the centre of Kassandrou, just a 10-min walk to Aristotelous square. The wifi is good: one of us worked from home (from the dining table) and had Zoom meetings 1 to 1 without any issue.
Sonia was always very quick to reply to our messages. The only issue we had when we arrived was the bedroom's wardrobe: one of the doors was loose. We told her and she sent a carpenter to fix it. We found a couple of croissants in the fridge and hotel slippers in the bedroom that were very appreciated.
We slept extremely well. The bed is comfortable, and the place is very quiet at night (during the day you may hear the neighbours). There are no traffic noises: the flat looks out into a "patio" between adjacent buildings. That's why you will receive most of the light in the bedroom: the living room window has the next building wall quite close (less than 3 m).
In summary: a great apartment to discover Thessaloniki.