We had a lovely time at Katerina's place. Both Katerina and Niko were very friendly and available. The hostel is snug and nicely furnished, we loved the little terrace where to have breakfast. The area is quiet but central, it was very easy to move around Athens from there.
It is true that the ad is a bit misleading though - the apartment is smaller than we expected, with a very small bathroom, and no real kitchen (although it is possible to fix something quickly with a microwave and a camping cooker). It is perfectly fine for one person or a couple, for a short period, but it is better to know this in advance. The only real criticism that I have is that there was some strong mildew smell in the house.