CityStay's a very clean, very secure, very modern hostel, and it's in a great central location in Berlin. Unfortunately, it suffers from one big common European hostel problem: it's often full of school groups. Fortunately, however, it does have a great bar and common area, which makes it easy to meet the other (minority) real backpackers staying there. Excellent value, and especially recommended if you're not after a crazy party hostel.
Rooms: clean, secure (they give you über-cool RFID credit-card-sized keys, which you wave to enter), warm and quiet.
People: too many school groups! And many of the other guests there are in big groups as well — big enough to keep to themselves. There are some proper backpackers here, though, so don't despair.
Staff: quite easy-going, and usually willing to help you.
Location: in Berlin's Mitte district, one block from the Alexanderplatz station and shopping complex, and walking-distance from key sights such as the Brandenburg Gate.
Food: the excellent buffet breakfast is meant to cost €4, but they don't enforce the price very strictly — I pigged out for free while I was here. Lunch and dinner also available, but it's a ripoff — plenty of better options abound nearby.
Internet: numerous PCs in the lobby for €2/hr, which is rather a ripoff, especially by Berlin standards.
Hot water: hot and reliable — although the whack-to-use and whack-to-keep-using water-saving taps are annoying. Bathrooms are also very clean and are numerous.