I stayed here for four nights (Monday - Friday) and had an excellent experience as a lone female traveller.
I did not meet any of the staff, however we exchanged messages about check in and check out, and they were prompt and friendly. Self check-in was very easy, the instructions given were clear and thorough.
I stayed in a self-contained room with private bathroom. It was clean and well furnished, with a very comfortable bed and good bathroom. It's budget accomodation, but what you get for the cheap price is very very good. It is a quiet property, back from the main road, with a lovely little court yard that was nice to sit in with a cup of tea after a long day.
The location is really great, with transport to the city center literally right out front (the 20 and 22 trolley car gets you there in 15 minutes). The 20 trolley car also goes almost to the airport (one easy change to a bus) which was really easy and convenient. Otherwise, its about a half hour walk to the city center, with many sight seeing areas along the way.
Overall, I had a really positive experience at Peter Pan Guest House, and I would absolutely stay here again if I visit Sofia again.