Mary's place is very clean and stylishly decorated. It's comfortable and cozy. I spent 99 days/nights here and enjoyed it more the longer I stayed.
It's in the heart of San Francisco. A short 10 min walk to the Powell BART (train) station. You don't have to visit the Tenderloin if you don't want to. The area is safe and vibrant. Lots of awesome restaurants around too.
Mary herself is great to interact with. You'll feel safe with her. She loves this place and it shows in how she takes care of her guests.