The blurb here on MacUpdate is less than helpful. Check the Monotype website: http://www.fontexplorerx.com/macfeatures/. There'll you'll find a list of enhancements and improvements in version 6. This is more than a maintenance release.
That said, I haven't felt the need to upgrade FontExplorer X since version 4. The changes in versions 5 and 6 are primarily for power users—which is not me. Not, at least, to the extent served in these versions. Still, I cannot blame the developer for endeavoring to add value to the application. There must be a market for these features somewhere. If that's not you, well, stick with whatever version you're currently using as long as you can. As far as I can tell there is no imperative to upgrade if you don't need the new features—or haven't upgraded OS X beyond compatibility with your current version of FEX.
By the way, while looking at the features list it occurred to me that I hadn't backed up the app in a long time. In checking the (manual) backup options I found that they are quite thorough. I recommend it.
In regard to compatibility issues, version 4 works with OS X through version 10.10 Yosemite. I won't have to worry about upgrading FEX until I upgrade to 10.11 or 10.12 (I'm currently using OS 10.9.5 Mavericks). You can find more information on the FontExplorer X download page at http://www.fontexplorerx.com/download/. Frankly I've found the website easy to use and most informative—quite unusual these days, where bells and whistles routinely impede usability.
On the site I learned that FontExplorer X Pro version 4.x has the widest range of compatibility, from OS X 10.6 through OS X 10.10—though version 3 also runs through five iterations of OS X, from 10.4 through 10.9.5. Check the web site for more info if you need it.
I've been using FontExplorer since the days when it was a free app from Linotype (since subsumed by Monotype). I've been happy with the app and paid for version one and every upgrade through version four. If FEX doesn't suit you for whatever reason there are other font management applications to choose from, many listed above.