I have stayed in the hostel twice, but many times in the bar on the basement.
The hostel itself it's ok, the beds are ok, but there are no lockers, and it's pretty dirty, so if you are in a private room it's ok, but sharing... the kitchen it's very small, and dirty, and the same wth the bathroom.The front desk people are ok, nothing too special. They have free internet, that is a plus and the location it's pretty good.
The best thing of the hostel is the bar on the basement, it's one of my fav. places to hang out in Sofia. Cool place with different rooms and areas, fussball, beer, cards, non-smoking area... and a great atmosphere.
