we had a very short but nice stay at Steffi's hostel. Although a bit far from the city center, it is definitely worth it with regards to the cleanliness, staff friendliness and facilities. Very good value for money, we'd stay again for sure!
I had a nice stay at Mingtown Nanjing Road Youth Hostel! It is rather clean, very well located and offers everything a backpacker would need (wifi, snacks and breakfast, common rooms, even toothbrushes and all). The staff is not especially nice, nor mean either. I guess it depends on you and the people staying in the hostel at the same time but as long as for me, the atmosphere was much pleasant and I had the occasion to talk with few people. I'd definitely come back!
Excellent location, very nice staff and rather clean guesthouse. It's not that secured, but I guess Stockholm's a safe city anyway.