This program has a hell of a lot of horsepower and features for a free program. I own and use Mindjet MindManager for mind mapping, and this free program has an amazing amount of the same advanced features, like filtering, notes, hot links to the web and docs, and customizable icons that allow you filter. Plus, the program is quite zippy (on my Intel iMac with Snow Leopard) even though its Java-based. (Maybe I shouldn't expect it to be slow -- Personal Brain mind mapper is also Java-based and it totally responsive with its live links and way-cool animated navigation.)
The biggest difference between this free program and something like MindManager is the overall refinement of the user interface, the look and feel, and navigation. For example, unless I missed it, I didn't see a way in FreeMind to hoist a node, or hide other nodes so I could just see a node I wanted to focus on, and I missed MindManager's nifty ability to "Fit Selection" to your screen. (In a huge mind map, being able to select a node and its children, and then hide others, and then have your selected node expanded or shrunk to "fit to screen" is a huge aid in working with your data. Again, FreeMind may do these things, but I couldn't find out how, if it does. I found the bazillion icons all around the FreeMind window hard to interpret, though each icon did have a hover pop-up -- after a while, I guess you'd get to know what the icons meant.
That said, of course one might expect a paid program of the price of MindManager to be way more refined and visually appealing, but in terms of pure functionality, FreeMind has the majority of features even a hard-core mind mapper would want. If I didn't own MindManager, I think FreeMind would definitely be my program of choice. Kudos to the open source community for making a remarkably powerful and useful program.