We just stayed three nights at the Radisson SAS in Berlin and have not regretted the choice at all. We chose the business rooms which was absolutely worth it since it came with some extras such as amazing view and a small balcony.

The location next to the Museumsinsel and Alexanderplatz is just perfect, buses 100 and 200 bring you quickly anywhere.

The room itself has a good size, though not huge, the same goes for the bathroom. The lobby with the amazing Aquarium has been praised numerous times already here and is really a nice attraction.

Service was friendly, efficient and the breakfast buffet was well appointed and tasty with mixed hot and cold food available.

The only minus points I would give to the wellness area which for a 5 Star Hotel should be better maintained, the steam room seemed mouldy. There was also no jacuzzy which many guests would appreciate after a long walk in town.

An absolutely fantastic Hotel! The Radisson is perfectually situated about a 10 minute walk to the Brandenburg Gate (and about a 25 minute taxi to the airport). The hotel is magnificent with and the staff extremely helpful.

Would thoroughly recommend this hotel in terms of location, presentation and the staff.

Chose the Radisson based on TripAdvisor write-ups. It was all we hoped for and more. The location is superb for sights as well as transportation options. The hotel is only a few years old, well maintained and stylish in a modern European sort of way. There is a huge fish tank in the lobby onto which our two double rooms faced. Even without the balcony, one could imagine being in the fish tank. The lobby is quite busy, but with the doors to balcony closed, you heard nothing. Rooms were a decent size, nicely decorated, and well designed, as were the bathrooms. The lobby has a nice bar area. There are several restaurants in the hotel, but they, as well as the breakfast/brunch, are quite expensive.
The bellboy who greated us was delightful and had many helpful tips. The staff at the reception desk were also extremely pleasant. The only staff member with whom we did not have a pleasant experience was the male concierge who often ignored us while he was on his cellphone, with what seemed not to be hotel business, and who was not particularly helpful and at least once downright rude. We did not let this influence our impression of the hotel.
We do highly recommend taking a walking tour with Inside Berlin. If all the tour guides are anything like ours, it will be a fascinating four hour tour. We loved it despite the cold, rainy weather. We also recommend lunch or dinner at the restaurant at the Reichtag where the food is amazing and the view comparable. We also had a great dinner at our tour guide's recommendation at Schwartzwaldstuben in an interesting neighborhood and excellent, well priced German food without tourists and great atmosphere. The chocolate shops that surround the hotel on either side should not be missed, especially the hot chocolate.
Steer clear of the restaurant Borchadt, which despite being recommended to us by friends, and despite having a German relative make the reservation for us in advance, had a rude manager, terrible service and mediocre food despite the high price with the US exchange rate. We were sent to a table in the basement, which looked like a basement, without our reservations being checked (even after protesting), and with open tables upstairs in the historical section.
Overall we had a great trip, taking in many sights, but did not cover everything in four days. Would have liked to add a trip outside the city.

We booked our stay for the Easter weekend in a standard room. The hotel is very centrally situated and has a wow factor in the reception atrium with its awe-inspiring aquadrome. This is normally a pay-to-see commercial attraction separate from the hotel, and it has the unfortunate disadvantage of constantly crowding the lobby with passing pedestrians having a free look from the hotel side.
Our room was adequate if a little small but was clean and well presented. Our stay entitled us to a bottle of champagne on arrival in the room, but this turned out to be a bottle of German sekt when it arrived. It took a telephone call from us to the manager before room service would replace it with the genuine article.
Dinner on the first night in the restaurant H-eat was less than expected. The food was disappointing in presentation and flavour and our main dishes achieved an unusual condition in that they were both over-cooked and cold. Our disapointment was compounded with the arrival of the bill €146 for just 2 courses and wine. Indeed one of the most consistent things with this hotel is its high prices for less than perfect drinks, food and service. Drink prices in the reception bar were high by comparrison with other 5 star Berlin hotels with which we are familiar.
Breakfast had to be queued for and it is not ideal to be given the option of breakfast in the reception atrium while being gawped at by tourists along with the fish!
One very unpleasant experience in both the restaurant and the lobby bar area was the mass of fruit flies that became a constant and annoying nuisance. Perhaps the hotel will try to remove the nuisance at some stage - certainly staff seemed not to care.
The main issue with the hotel is it's 5 star status. I would question any establishment who uses this label while not providing basic 5 star amenities in rooms. No bathrobes or slippers, no mineral water, no CD player, one towel per guest per day, no turn-down service, and poor quality toiletries and absence of simple items such as cotton buds, nail files and hair conditioner. Needless to say, all higher-price rooms offer these but as the place isn't cheap to begin with and it claims to be 5 star why the difference between this and other 5 star operations in Berlin?
I do not find a large fish tank sufficient reason to go back and if I ever see another fruit fly it'll be too soon!