I've used it on and off for years and years. I used to do a lot of mappy-visualization work and thought they had a pretty innovative concept going, with a great implementation.
The great thing about The Brain is that you don't have a limited amount of space - it's very easy to navigate around a very large structure because they've cleverly designed it to hide things that aren't linked to the current item. So for some people it's perfect - you can have the whole shebang in one map.
Mind-mapping is useful partly because of the context - you have a structured page and everything links off a central concept etc etc. Where The Brain might not do as well is if a visual person wants to see more of the picture at a time. Where the mind-mapping falls down is that each page is limited in size because you see the whole thing. Both work, and both sets of limitations can be solved with effort.
I'd say play with the free versions for a while - it's up to how your customer thinks and which they feel more comfortable in.
There's a trick with The Brain - in the options somewhere you can turn on "distant thoughts" and that lets you see an extra node away. Also a personal choice, I prefer it because it gives me a bit more context.
