We stayed 6 nights at Sandra's place and everything was great.
Flat is small but good for two people with all the equipment that you can possible need during your stay. The place was renovated recently and you can easily feel that since it looks as brand new! Sandra was very helpful with the check in/out process and always available to assist us during our stay.
The flat location is great, you can easily walk the whole city centre although parking can be a problem- but this is a problem in Thessaloniki's city centre in general!
I highly recommend this flat and I will definitely use it again for my next visit to Thessaloniki.