It's rare that I give perfect marks across the board, but Curiocity earned it! I booked 2-weeks at Curiocity via Hotels.com, and they put me in their sister hotel (12 Decades) right down the street. From what I gather, Curiocity is the hostel and 12-Decades is the hotel.
I was absolutely impressed! The staff were great, the hotel itself was awesome (it is an art-themed hotel, and each room has a different motif), and the neighborhood is wonderful. Maboneng is an ultra-hip part of Johannesburg with great cuisine (PataPata, Ravioli, Soul Souvlaki, Bertrand's cafe) and a really cool vibe. I'm a bit of a straight-laced guy, but even I loved the bohemian feel of the area.
The only con was that the wi-fi was sometimes difficult to connect to.
I absolutely recommend Curiocity, and would stay there again in a heart-beat.