For a short stay, Nancy's flat would be perfect for a backpacker looking for a cheap room in the heart of Mong Kok. However, If your looking for something a little more comfortable for a longer stay, I wouldn't recommend the apartment due to its cramped and awkward facilities.
I felt the photos misrepresented the cleanliness and general appearance of the apartment. The walls were scratched and stained and the floors in the bedrooms were dusty. There were holes in the brickwork and exposed pipework on the inside of the kitchen door. The general appearance of the apartment felt like an afterthought. For example: lamps that were un-plugged and unusable and plastic house plants tipped over in the corner of the living room.
The shower was mounted inside a small toilet / wet room and was awkward to use due to its multiple functions. I found my bed comfortable and slept well, I didn't find the complaints of noise to be a factor, but be warned that if you are using the AC, closing the curtains causes condensation which means water dripping all over your pillow and bedsheets. If you have mobility issues, or are overweight you will likely struggle with the entrance to the apartment, as it's very narrow metal gate.
The apartment is very close to Mong Kok MTR station and a bus route that takes you to the Airport. It's a good location if you want to avoid the shopping centres and commercial areas further south, but still wish to visit the markets near Mong Kok and Temple Street.
Nancy was very direct with her communication and answered any questions I had promptly. The PDF she provided with details of how to reach the flat were the most detailed and accurate instructions i've had while using Air BnB.