Nestor is a welcoming host who always responds quickly and helped us whenever we needed it. The appartment was clean and had all the amenities we needed (most notably air conditioning, which we were happy to use after a hot summer's day in Athens). Furthermore, Nestor has a list of his recommendations for everything in Athens - tourist attractions, restaurants, bars, etc. - which is great when you need ideas on what to do or where to eat.
The neighborhood may seem sketchy, but we had absolutely no problems (even walking through it very late at night/early in the morning) - and the sketchiness of the neighborhood's buildings does not at all match how well put-together Nestor's appartments are. On top of that, you are truly at the center of Athens, and so most places you'll want to go to are within walking distance.
Overall, I would highly recommend, especially considering the modest prices!