My 5 year old girl and 3 year old boy share a room and they love it. The reason we initially put them together was our eldest was having nightmares, and was scared to be in her room by herself. After I slept on the floor a couple of times when she was really upset, we asked her if she wanted her brother in with her and the enthusiastic response was yes.
Since then we have had few problems with nightmares and waking as the very presence of someone else in the room is comforting to them. We do still get the occasional night time visitor but it is not often.
A small problem is that they can talk and play in the room and keep each other awake and we put them to bed a little early to allow for a settling period but otherwise there are no problems.
A major bonus is this has freed up a room for visitors or one of us to occasionally sleep in if I am snoring or one of us is sick.