My family stayed at Hotel Nikolai Residence for just about two weeks over Christmas and New Year's Eve. We are a family of four - my husband, myself, our 12-year old son and 16 year old daughter. We knew we'd be there a fairly long time and wanted as much space as possible so we rented the largest available room, which allowed us all to have our own space. It was very clean and very comfortable. I was very happy to find that there were more than enough pillows and blankets. Housekeeping came daily and did a great job. We checked in after reception hours, and it was seemeless. We stopped in at the reception desk the next day and the man there was very friendly and helpful.
The area is absolutely ideal. If you're looking to sight-see in Berlin, this is the place to be. Many highlights are in easy walking distance, and public transit in Berlin is excellent. Many great dining options, and Museum Island is a 5-minute walk away.
The rooms are well insulated against noise. We were there over NYE and despite the constant fireworks all night, the sound level was very low.
Highly recommended - I'd stay there again