I stayed at the Meininger hostel on Tempelhofer Ufer over Easter 2006 and again in ealy August 2006.
Both times I was travelling alone and booked a single room.
First time I had a ground floor room which was very easy to access direct from the street, without going into the hostel proper. This room had one single bed.
Second time I was on the second floor (two flights up) in a larger room with two single beds.
The rooms are very modern, if spartan. Cleanliness is excellent.
For a continental breakfast the selection is so-so: cereals, toast, cheese, cold meats, yogurt, fruit, juice, tea and coffee.
The service was at all times fantastic - very helpful staff.
There are three Meininger hostels in Berlin; I chose this one because it's the smallest and cosiest, and all rooms have private bathrooms. The main hostel, directly over the canal, is massive with much more of a typical hostel vibe.
The location is excellent - about a 10-15 minute walk to Potsdamer Platz, and about 10 minutes to the restaurants and cafes of the southern part of Kreuzberg.
I'd definitely stay here again.