We stayed in the Arba suite at Riad Diana for a week in December 2012. The hotel is very nice and Boujamaa and his staff are really friendly and helpful. This is a small and peaceful hotel, located about five-six minutes walk from the square, Jemaa el Fna. The breakfast was good and it was nice to spend time at the roof top terrace.
We booked an excursion to the Atlas mountains through the hotel and it was very good. But if you do that, chose another restaurant than the one the driver will suggest. It was rather expensive and made us feel a bit fooled in the end. It feels like the driver has some kind of agreement with the restaurant, but just chose another one.
The cleaning of the rooms are the least good about Riad Diana. For example the toilet wasn't cleaned when we arrived the first night and the doors to the bath room were dusty. But if you are not satisfied with something, just talk to the staff and they will do their best to help you. Some of them speak English, some only French. Shampoo, shower gel and hair dryer are availible on request.
If you go to Marrakech, I recommend you to stay at Riad Diana.