TikiWiki is one cool product. It's incredibly configurable, customisable and extensible (and many other buzzwords besides). You can get it running in about 10 minutes, but also spend ages getting it "just the way you want".
TikiWiki is way more than a Wiki - it's a "one size fits all" application that includes countless other features, like forums, FAQs, articles, etc etc. It's a "next generation" application that relies extensively on a backend database to store configuration and data. That makes it a degree more complex than something that uses config files, but it's easily worth it. The fine grained permissions model of Tiki means that you can assign specific abilities to groups of users. Also, the feature enable/disable capability means that you can switch off bits of Tiki you don't want, removing the need to maintain permissions or content in them.
The layers of Tiki application (PHP, Pear, Smarty, MySQL etc) make it quite slow. On a pretty much dedicated machine of reasonably decent speed, pages were getting created in about 2-3 seconds. That doesn't sound too great, seeing as it's not scaled up at all. However, the addition of PHP Accellerator gets those times down to about 0.5 seconds. Incidentally, PHP Accellerator is another great application, which is simple to install (unzip it, add one line to php.ini, and that's all you need to do).
All in all, if you've got a web site, and you want to provide numerous interactive features, then Tiki is really good. It's got to be worth a few minutes of your time to try it out (there's an online demo, or try it yourself in just a few minutes).