I skimmed their web site, and from the features list, they've got some under the bonnet features that are useful, but not immediately visible, such as a trust system.
I'm not immediately impressed with the user interface, but I am willing to accept that this may more a matter of what I'm used to than anything in particularly wrong with the system. The one thing which seems a problem is that there's (I am told) only one level of grouping of topics.
That means we either have to
1) live with a flat hierarchy, where it's difficult for a user to focus on the bits they are interested in.
2) treat the lower level topics as threads, .e.g. all posts on mammals go in a single Mammals threads
3) set up an index page outside the forum to record the hierarchy (but any integration with the notification system would be non-trivial)
[Though since it's open source there's the possibility of changing this. On the other hand, once upon a time some colleagues asked me to help out fixing a bug in a compiler. I knew immediately what was wrong, and what had to be done to fix it, but it still took us 2 or 3 hours to find the code. I suspect that it would be harder this time.]