We really enjoyed our time in Madrid staying at Susana's place.
Good things:
1. The apartment is in an absolutely great place for walking to all the main sites, going out for food at night, and partying later if you want. Yet the traffic noise is minimal, with only a little noise from nearby flats or bars
2. Susana went out of the way to look after us. For example, she arrived an hour early when I arrived earlier than expected. She also arranged an early taxi in advance for my 71 yr old mum.
3. The roof terrace was beautiful in the autumn sun.
4. The apartment is safe with 2 deadlocked doors.
Disadvantages
1. The standard of the apartment is reasonable - but maybe the quality is not as high you might expect for the price. (Eg beds, linen, bathroom etc). But I understand that to have a flat in a perfect spot, you have to pay more.
Overall:
I would absolutely recommend Susana's place as long as you're not expecting luxury, and you want to be in central Madrid. Both me (visiting from Australia) and my mum (visiting from uk) really enjoyed our time.