Chuck Frey (author of The Mind Mapping Software Blog) has created a valuable resource containing various Research & Reports which I highly recommend you visit. It contains surveys, reports and eBooks available for download.
Web-based mind mapping applications
Benefits of mind mapping software
Applications of mind mapping software
A comparison of the major web-based software applications
8 Ways to Improve Your Mind Maps
10 Mistakes to Avoid with Mind Mapping Software
Power Tips & Strategies for Mind Mapping Software
The Mind Mapping Manifesto
I reviewed “Power Tips & Strategies for Mind Mapping Software” in October 2007.