To start with, I'll clarify by saying I stayed at the Grand Hyatt using hotel points. However, I have since found out it ranks along with the Ritz Carlton (across the road in Potsdamer Platz) as one of the 5-star hotels in Berlin.
The decor is ultra modern and the hotel is brightly light. Check in was very smooth and the clerk took the time to walk us around the hotel and to our room, pointing out the various amenities. She was very friendly yet professional, and her effort was appreciated.
The room itself was quite nice. The best aspect was the brightly lit, walk-in shower area. The bathroom itself has a small LCD monitor which is in sync with the TV in the bedroom, so you don't have to miss your favorite show or movie even if you're in the bathroom. If you have access to the Grand Lounge, complimentary breakfast is provided with a variety of juice and coffee. The lounge is also open until the evening for non alcoholic drinks.
The hotel is located in Potsdamer Platz, one of the areas which has been redeveloped since Germany was reunified. Across the road is the Sony Center which is worth checking out for its architecture. It is a 5-minute walk from Potsdamer Platz U-bahn/S-bahn station, and is also within walking distance (about 20 minutes) to the Brandenburg Gate and the Reichstag building.
Tips: There is free Internet at this hotel. It is located to the left of the front desk, where 3 desktops are available for browsing.