I have found they are a useful tool for group brainstorming. It helps provide structure to the unstructured thought process of a group of people without really constraining their thinking in any particular direction. However, you need a very strong group leader to make sure the map represents the ideas of the group and not just their personal ideas.
I use them off and on. They are particularly useful for illustrating information architecture. You could apply this in site navigation, development, or just studying for an exam.
As humans, our mental models follow a hierarchical nature that help us create and understand relationships in implicit, visual, and/or spatial ways that come pretty naturally.
