I stayed in Mamaison Residence Diana Diana for a week in mid June 2014. When I have to work in a city for more than 2-3 nights I always try to book an apartment as it gives you more space and flexibility with meals. My experience was very good from the booking on their own website until I left.
About a week before my arrival I was contacted by the hotel if I would like to use their pick-up service at the airport, which I did and it was very well organized and comfortable.
The apartment (one-bedroom suite) was spacious, modern interior design, kitchenette with cooking facilities, separate lounge and sleeping area, bathroom with toilet and a second separate toilet for guests. I worked comfortably at the table, but there was a desk as well. Wifi was really high-speed and worked without any problems.
The location was very quiet although it is right in the city centre. You go through a gate at Chmielna street, where you have a lot of shops. But as soon as you have passed the gate the inner court it is really quiet and I did not hear any noise in the evening. Walking distance to the Palace of Culture is about 10 minutes. There is a busy restaurant downstairs with a nice lounge terrace, Restaurant Maka i Woda. I am not sure if it belongs to the hotel, but you could enter it from inside the hotel. I had some awesome good Italian like dinners here.
Mamaison Residence Diana is at the top end of my experiences with city apartments. I have stayed in several 5-star hotels in Warsaw. But after having experienced Mamaison Diana Residence I will definitely stay again in this aparthotel when travelling to Warsaw. It is an awesome place and extremely good value for money.
I have only one advice to the hotel management: you have to do something about the bins at the back side of the hotel which are seen from the window. It destroys the good impression of your hotel.