I've been to Berlin twice in the 10 months. The first time (vacation), we stayed at the Berlin Marriott using points from our Marriott Rewards account. The hotel is truly outstanding and is located near Potsdamer Platz. Without points, it's admittedly quite pricey. I realize that you are probably looking for something less expensive, but I mentioned it in the event that you belong to any of the hotel chains customer rewards programs (if you do, definitely look at their properties in Berlin; it can save you 100s of Euros). Last month (on business), I led a group on a study tour and we stayed in a much more modest hotel (the Ibis Berlin City Potsdamer Platz; located on Anhalter Strasse, the street behind the 'Topography of Terror'). The Ibis chain is pretty bare bones (room and amenity-wise) but every one I have stayed in has been extremely clean and had a good buffet breakfast. The staff were also very friendly and helpful. The location was excellent - easy walking distance to many of the sites in Mitte.