This is actually a topic of much debate on the Dis Boards. As Favorite Auntie mentioned, it seems that some people have had difficulties with the cooling units in their rooms. Others have not.
In four cruises I have never had a problem with the in-room cooling units. We always pack 6-8 cans of Coke in one of our carry-ons just to have some soda in the stateroom for the cruise. We always place this in the fridge right after we get to our room and by the next morning it has always been cold.
On our cruise in May I plan on taking an appliance thermometer to measure just how cold these units get!