Despite what many people may think, simulators are one of the best ways to advance your flying. I have spoken with numerous 3D masters pilot and everyone has said they make frequent and good use of a simulator, even some of the worlds best such as Duncan Osbourne!
The reasons for this are that they:
Enable you to focus on maintaining and improving your orientation skills
Develop reflex reactions in unplanned and potentially expensive situations
Enable development of manoeuvres. These can either be existing ones you are currently unable to fly or even manoeuvres as yet undocumented.
However, development of the above are dependant upon a decent simulator and proper use of it. Below I will describe what I feel contributes towards a decent simulator and what entails proper use.