I've stayed at Lunda Pensionat twice (so far). Lunda Pensionat is a pretty small place on a quiet street in Södermalm. The location is very good, with a T-bana stop very close to the hostel. I found the hostel very quiet, pleasant and clean. The views from the 6th floor rooms (with shared bathrooms) are amazing. The staff are very friendly and helpful. There is a kitchen you can use and lots of cafes and restaurants near-by. The prices are very good for what you get. I would stay again and recommend it to others, too. I've only had a little problem once when my keycard didn't work, but someone came down to open the door for me and gave me a new keycard right away. :)
My only "real" complaint is that there seems to be no way of paying ahead of time with credit card (as I like to pay for hotels and flights before I travel).