My sister and I stayed at Sandy's apartment for 4 nights. Sandy was very quick to reply to all of our questions, and even had a list of restaurant recommendations. The apartment was very clean, modern and comfortable. The biggest bonus for us were the lockers. My sister and I went hiking on mt. Olympus before our stay at Thessaloniki, so it was extremely convenient that we could drop off our excess bagage at the lockers.
The bed was very comfortable, we even had 2 pillows and we were very grateful for the airconditioner :)
Concluding: easy communications, comfortable apartment, central location and bonus lockers make this the perfect apartment to stay! Would definitely stay here again, especially since we (unfortunately) didn't make it to Mytikas, so we'll need to return for that ;)