(Not wanting to sound like a 12-step program...)
I've been using DokuWiki for at least 3-4 years, so I'm hooked. It's simpler than Trac (which was much less mature then, too), and none of the sites I've used it on have needed anywhere near the nuclear power of MediaWiki. Most of the time, in most ways, DokuWiki fills that happy medium quite nicely.
I've deployed Dokuwiki internally at over a dozen different clients, particularly for information capture within software/Web development groups, internal and external service organizations, and so on. "If you didn't write it down, or can't find it after you did, it didn't happen the way you remember it." Of course I also use it on my own systems, to track development projects, system maintenance procedures and histories, network configurations, and so on. I've even poked around at drafting paper and book manuscripts with it, but still find DocBook a much better fit for that.
I've been known to poke around the edges of things, though, and sometimes slice my fingers pretty badly. I've put a couple of
feature requests in, and now think I've found a
serious bug with interaction between lists and tables.
I'm glad to be a part of the community.