If you try to describe what an ideal hostel should be like, pretty much every point will be met by this place.
Amazing, friendly, sociable staff? Yes.
Open-plan bar area open until 03:00 making it easy to socialise with travellers? Yes.
Large comfortable bed with blackout curtain for privacy? Yes.
Huge locker per person that is easily big enough to hold all your stuff? Yes.
Individual plug socket and lamp for each bed? Yes.
Great location, central in the city? Yes.
Free walking tours arranged? Yes.
Very well priced? Yes.
More positives, but there's so many I can barely remember them all? Yes.
If I really scrape the barrel, the only downsides I can think of was that there was no chalk at the time for the pool cues and the big world map downstairs is from 1976 so it's still showing places like Yugoslavia and the Soviet Union. It's still an impressive looking map though.
Oh, and one final point: OK Hostel is a very misleading name. It is much better than ok. It's amazing.