I highly recommend Marily's place. The location is unbeatable - right in Monastiraki with 24/7 markets, spice shops, cafe's and bars right down the street. The metro station is a 5 minute walk away.
The apartment is a good fit for 1 person or a couple. It was nice to have the extra bed/couch. The balcony was with lots of space.
The bed was very comfortable and had 4 pillows with a warm blanket. The wifi was very good, the kitchen had all the supplies you need, and there were plenty of desks/tables. The wardrobe is quite large also.
The best part, is Marily's kindness. She gave thorough instructions when I checked in, and stayed in communication with me the whole time. If you are looking for a place to stay in the heart of Athens nightlife and restaurants, for a good price, and within walking distance to landmarks, you should go to Marily's place because most other places are quite expensive.
The only thing I can say is the walls are thin, but in the winter there aren't many tourists, so this wasn't a problem for me.