I stayed here for a week having just arrived in Germany and enjoyed it, although there is definite room for improvement. Firstly, the staff are very helpful and knowledgeable. I arrived before the scheduled check in time and they were happy to make me a coffee then look after my bag while I went back out.
The location is OK, but a 45 minute walk to the train station/cathedral. There is a metro stop just outside which takes you there quickly, but also a lot of shops and restaurants closer to the hostel. With the window open at night it is extremely loud due to continuous traffic outside and too hot in the summer to keep it shut, so not one for light sleepers.
There is no wifi but a free computer, although that is very busy with everyone using it (or McDonald's wifi directly downstairs). The atmosphere in the common area/kitchen is very good, there's a guitar for people to use and rooftop terrace where everyone socialises. I was travelling alone and instantly met other people each day. The free breakfast is basic but OK - toast, spreads and cereals and there is free spaghetti every night for your use.
The rooms are decorated by a theme (dragons/Africa etc) and are clean with comfortable bunk beds. I'd stay here if I were to return to Cologne.