I've been testing and following Ninjaboard since the earlier beta releases and have enjoyed watching it develop. Installation has always been a breeze, importing data from other forums like Kunena couldn't be easier and the admin interface is both clean and not overpowering. Don't be caught off-guard by the seemingly simple admin interface, it's noting less than advanced and doesn't try and impress you by overpowering the layout with all of the options presented on one screen.
The Chameleon theme is a designer's dream. It is a true self adapting theme that blends itself nicely with your site template. It's power not only lies in its visual abilities, but also allows you to optionally set custom code (tags, classes, ids) to be used for some of the forums elements, such as the cancel button.
The Usergroups settings rivals the power and flexibility of Joomla 1.6's ACL. It is an amazingly powerful access control feature that allows you fine granularity. Most users won't need to touch it, but it is good to know that it's there. Plus, the user interface for mapping Joomla user groups is really cool.
To me, Ninjaborad is the forum of the future and I'll be using it everywhere I can.