We (couple) stayed for 5 nights in Art'otel Berln Mitte, which is a fine hotel although it doesn't fully live up to it's self-proclaimed selling points. The art/design factor is no higher than in other hotels in this price range. The quality of the rooms (we had the suite) is as you might expect: clean and well equipped. Service is good and friendly.
The location is very practical: although the neighbourhood itself is rather quiet, it's centrally located between everything you might want to see in east, west and Mitte. We rented bikes for a couple of days which proved a very nice and fast way to explore the city.
The hotel has a couple of very nice assets, such as the courtyard and the historic part on the riverside.