I've tried, I believe, every mind mapping app for OS X. I like NovaMind the best for a simple reason: speed of input ability.

I can enter information nearly as fast as I think it. While OminGraffle and some of the others may offer every concievable option, I spent more time fiddling with lineshapes and such instead of just mapping.

The only complaints I have about Novamind are that it is looking a bit bloated. When you open all the available viewers, it overwhelms your screen (reminds me of Omnigraffle which seems overwhelming to me). It would be nice to optionally dock some of the viewers in the preferences and to try and simplfy where possible.