Initially I hated them because the concept of letting anyone edit your web page is repugnant, to say the least. I came to accept that, in certain cases, they could be useful as knowledge centers. If you don’t like writing documentation, or if you don’t know every intricacy of the subject matter, wikis can be great for letting the community help to spell out a set of articles.
Now I hate them because most wiki systems are awful. MediaWiki, the most popular, has feature-itis to the extreme. “Someone has a fetish for an aberrant magic word that only one site on the entire intarweb will use? Sure, we can add that!” The interface becomes cluttered with strange options that 99% of sites will never use. And God help you if you try to learn all the magic words, templates, variables, and other things so complicated I couldn’t even find a help page for them. Which says a lot in itself. If you need everything and the kitchen sink, it’s your choice. (Wikipedia is only acceptable because it has huge wealths of information, and because MediaWiki was built for it.)
Most other wikis are either fugly, undocumented, unmaintained, or lacking crucial features. However, in my travels, I have found a few very nice and innovative wikis. Here they are, in all their glory:
Instiki: the original Ruby wiki, so easy to install and use you’ll wonder if it’s really a wiki. Two steps to install, easy formatting, and it still has all the important stuff for a basic wiki. Plus, it was the inspiration for Rails.
Trac: fantastic not for its wiki abilities, but for tying the wiki into the software development process. It has a subversion code browser, wiki, milestones, etc. Take a good look at it if you want your project to have a home.
Tomboy: I almost used Gnome for this program alone. It’s an awesome little system tray utility that gives you a personal wiki for notes, to-dos, etc. The quality is in the simplicity. It’s always available.
Wikalong: an extension for Firefox that adds a wiki in your sidebar, linked to the current URL. Handy way to keep notes on certain pages, share information, etc. You could also post your Adblock filters.
There was a wiki I found at one point that was good for software documentation. I can no longer find it, but it might have been DokuWiki.
I have a suspicion that many people use (other) wikis when they don’t feel like doing design. That is to say, most wikis are butt ugly by default.