I think it's important to keep in mind the different effects that one might achieve using mind mapping, with regard to the way mind maps are being evaluated in the literature mentioned so far. While retrieval ability is easy to measure and certainly a useful metric that approximates one important aspect of learning in an academic setting, retrieval isn't everything.
My own experience (n=1) is that mind mapping is probably not a great use of time when preparing for the kinds of evaluations usually required of one in college. Mind mapping has, however, proved remarkably beneficial to my ability to index information, and to see relationships between different things. This is crucially different from the retrieval model described earlier: I am not trying to prepare myself to reproduce some given body of facts; rather, I want a tool to help integrate new information with what I already know, or else help explore the connections between disparate elements in my knowledge store. For these pursuits mind mapping has been a superb tool.
I suspect that the benefits of mind mapping may be related to the benefits of the Creative Uses Task described by Chrysikou (2006), for the same reasons I described in my answer to this question, namely, the elaboration and schematic connection between far-flung semantic contents. How is the industrial revolution like an old-fashioned donut? I have no idea, but framing a context that meaningfully interprets both things can elicit a more profound understanding of the logical path between them. Further, this process has more than once been the cause of an insight, or reorganization of conceptual knowledge, a la Durso et al. (1994); less personally, it's a flavor of combinatorial activity that features heavily in many models of creativity (c.f. Thagard & Stewart, 2011).
Chrysikou, E. G. (2006). When Shoes Become Hammers: Goal-Derived Categorization Training Enhances Problem-Solving Performance. Journal of Experimental Psychology: Learning, Memory, and Cognition, 32(4), 935–942.
Durso, F. T., Rea, C. B., & Dayton, T. (1994). Graph-Theoretic Confirmation of Restructuring during Insight. Psychological science, 5(2), 94–98. Association for Psychological Science.
Thagard, P. (2011). The Aha! experience: Creativity through emergent binding in neural networks. Cognitive Science 35, 1-33.