A site called Innovation Tools (which I recommend) published in September 2006 the results of a survey on mind mapping software (in PDF). MindManager is the clear leader, with ~70% usage, with the open source FreeMind at ~10% (which I tried before prefering MindManager).
This survey is especially interesting when you see the diversity of topics people use this kind of apps for: to-do lists, preparing presentations, taking notes, solve problems, plan projects, decision making, etc. According to the survey, the single most important benefit of using mind mappers is "Clarity of Thinking". This is very true, and you can get results very quickly. A simple map can take me ~15mins to create, and usually the result can be used directly in your work (for example, for the titles of a PowerPoint presentations, or the chapters of a word document, or even a database structure or class diagram).