The apartment is good size - can fit 1-2 people. Safe area. You can walk to the nearby Takadanobaba Station which is also a JR station so it is convenient if you have a JR pass. There is also the Waseda station nearby. Lots of restaurants around, including the popular Ippudo ramen. Though location is convenient to the stations, you will likely need to take transportation (or walk 30-45 min) to the main "action" in Shinjuku as this location is not as central as I would have liked it.
The inside of the apartment is more cluttered than the pictures, which made the space feel smaller and unclean. For example, there were suitcases lined up against the walls with notes saying do not open. There were also additional furniture/cabinets (including an old refrigerator?) with signs that said do not open/use. Overall it felt like someone quickly left the apartment and left their things lying around. Older furniture, tape in the shower, and used dish towels made the place feel unclean.