there is a highly engaged community of users, supporters and developers which makes this extension one of the best you can get and for sure by far the best forum component for joomla on the web. forget about all others!
have read many of the comments and i have to say some aren´t true. f.e. there area tons of templates for kunena. i personally use a dark template by yootheme for my joomla and so i needed a dark theme for kunena too. it tooks me about 5 minutes to go to the kunena forum and find a bunch of templates. i now use a very elegant template by kenlapz and it´s absolutely great. installation was as easy as it could be.
kunena itself runs stable. installations and upgrading is a breeze. i never had any problems with kunena. i experienced some smaller bugs but the developing of kunena is very very good and fixes are avaiable in no time. also they have a great wiki which provides a lot of information for modifying and customizing your kunena.
i tried out sevral other soulutions and i have to say this is by far the most professional forum solution on the market you can get.
with some addons and modules it even beats vBulletin, SMF and phpbb.
i really dont´know why those peepz give away this fine peace of work for free. this product would deserve it to be paid. however, there are only 5 stars to rate. this one deserves it really. the first time i gave this rating.