Visited Sofia last summer for a few days.

Its a great city, but lacks in attractions for more than a i'd say four day visit.
It is still relatively cheap if you dont eat at hotels.

Security wise, I had no issues, I was more afraid for my belongings in Greece and Italy. However, at bars, male alone does get attention from the "ladies"

Make sure to visit the Rila Monastery, a short day trip from Sofia