A great stay indeed. I had to head into Marousi regularly and the cost of Mary & Theo's apartment plus a taxi to Marousi and back, was much less expensive than getting a place next door to where I needed to go.
Don't be fooled by what you see just outside the apartment building (which initially I thought looked a smidge grotty with some big bins on the street opposite). Inside the building the apartment is very nice indeed. It was a super clean place filled with sunlight and a great view that I enjoyed seeing the sun rise and set to everyday. You can see the hills and the city in the background.
There is also a huge balcony. There are loads of local shops & conveniences just around the corner or a short distance away. 4 mins walk max and you are by the square with a bunch of shops and a local taxi rank.
By the way, use local, regular taxis rather than Uber, Theo gave me this tip and it is true. Just make sure they have a meter. If the taxi driver says the meter isn't working he's lying and trying to scam you. So firmly say "no thanks", then get out and go to another taxi in the rank. If you keep walking down the square to the pedestrian only area there is vibrant area with a long, central lining of trees where there are lots coffee shops and restaurants for you to enjoy. This place is definitely a hidden jewel.