Not much to add to what others wrote. Well decorated, clean and just-the-right-size apartment. The kitchen is well-featured. While you may be a bit intimidated by the location, specially at night (i.e. some old buildings and graffiti), i can assure you it is quite safe and i never had any issues during the two weeks i stayed.
The apartment is well-located and a couple of minutes walking from monastiraki and center. There are also a couple of nice restaurants and places to eat around the block (e.g. Elvis for some quick souvlaki). You also have a supermarket very close to the apartment which is quite handy.
You may notice a bit of noise from the street but nothing to lose sleep over. During the winter, the apartment may get a bit cold but it has a good AC so you should have no problems.
Maria is handy and usually pretty quick at answering questions or concerns. She let me be on my own which is something i like and expect from hosts (i.e. i like my privacy). Definitely staying over again if i ever visit.