I have a small set of criteria which I use to choose amongst the mind map tools available right now.
Entering information must be fast, as must the process of reorganizing it. I'm prepared to sacrifice high-end features in order to achieve this.
Since this is a tool which is used to capture information, it must be available on as many of my personal devices as possible, sharing information between them, preferably automatically.
It's rare for a mind map to be the final form that a piece of information will take. Rather, I use them to capture and process information before using a different tool to turn them info the finished article. For me, that usually means text or diagrams. As such, it's important to be able to take information out of the mind map, either as a plain text outline or a standard outline file format such as OPML. The ability to export as an image or PDF is also useful for those occasions where I want to take advantage of the efficiency of a mind map to share information with others.
MindNode by IdeasOnCanvas matches these criteria very nicely. A close second is iThoughts, although I find that while the latter is more powerful, it's not quite as intuitive and synchronization isn't quite as seamless as MindNode.