Casa 48 was a great a great place to stay at to be honest. At the beginning i had my fair share of concerns regarding its location but I always felt safe, even when going down the favela.
It has amazing view, I couldnt meat Alex but Valentin was there and he was so very helpful - explaining general directions and tips for Rio, safety aspects, and even daily places that the guests can go to. You can meet also quite interesting people in the hostel and share travel stories with each one of them.
And DONT MISS THE SUNRISE from the balcony - it is worth it!