DokuWiki is a wonderful wiki system to use. I love the fact that it does not require any database at all to operate. It has a great module installation system, takes two seconds to add a new script or feature. The wiki syntax used is very simplistic, and easy to understand. It comes pre-loaded with tons of great content explaining about the wiki. Installation is a snap!
My personal favorite feature, however, is the backend authentication. If you choose to, you can actually let your users of a forum or other php based web apps (comes with these features already loaded in!) use the wiki as if they are logged in on the other app, it actually utilizes the cookie. You may have to change a few config files around to use the backend auth features, but mostly just to change the file path.
I have only experienced two glitches. The first one is a writability issue, the ACL settings do not take automatically. If you have a file set to any chmod which does not permit read or write for php, you cannot edit certain pages. Not a big deal, just set chmod to 777. My other glitch was sort of a big one, occasionally all the text on the button disappears until I restart my browser. The buttons are still there though, so it is not a terrible problem.
Overall, I am very satisfied with the quality of this product.