I find them very useful for shared documentation. I've used two regularly as a content producer, and several regularly as a content searcher.
The two I've contributed to are my current workplace's (which we're using to document things I do regularly - like producing stats, specific reports, etc - that other staff do far less frequently. I can post and update instructions as well as the report files I use, and librarians on other campuses can easily grab the files and information without needing to send me an email or question.) The other was at St. Kate's while I was working there in January, where they were using a wiki for documentation for public services staff (particularly useful since they have new reference assistants every semester. Knowing things like where the paper is kept, or where to get more staples is very useful.)
What do I use? Wikipedia, for a quick overview of a topic, and to get an idea of where I'm pointed. While I don't trust Wikipedia information for final facts, I find it extremely useful to learn specific terminology for a topic, get an idea of major controversies, and often the links and references cited turn out to be a good place to get started. (I find it, unsurprisingly, most useful for extremely current material, like breaking news stories, or for online-related topics.)
I'm also an online gamer: I use WOWwiki (for World of Warcraft) to figure out how to solve quests, get from one area to another safely, and other game questions fairly regularly. As a casual player (I'm lucky if I have time to play 5-6 hours during the week, and it's often less than that), things like the WoWwiki and other player documentation go a long way to making my play time more fun and more productive.
The question of Wikipedia:
I got asked to do a really interesting comparison while I was working at St. Kate's in January - compare entries in the Online Britannica and in Wikipedia. This was meant to be a casual study - pick five topics and review them, but I did pick 5 different types of content (history, science, technology, popular culture, etc.) and compare them.
I found that Wikipedia was, on the whole, far more useful. The one that stuck in my head was looking up Anne Boleyn. Britannica's article was about 500 words, didn't mention many of her family's names (making additional research start points more challenging to find), and the only reference work cited was from the 1970s.
In contrast, Wikipedia's article was several thousand words, and not only walked briefly through major historical points, but helped put her family in historical perspective, and gave a brief overview of major recent scholarship. One of the reasons I picked Anne Boleyn was the recent attention due to the Other Boleyn Girl book and movie: the Wikipedia article also provided links to criticism and commentary on the book's accuracy or lack thereof when it came to common understandings of the history.
The down side, of course, is that there's a lot more information to wade through: it's easy to get caught up in minute details that aren't actually relevant to your goal. It's also painfully easy to get distracted by some random intriguing detail that isn't actually what you were looking for, follow links, and look up half an hour later wondering why you're now reading about how names are chosen for new moons, when you started out looking at Greek history.
One thing I actually like about wikis is that they require you to pay close attention to what you're reading (assuming you take your research seriously) and to use your common sense. Being aware that anyone can edit them, and that the information needs to be confirmed elsewhere helps keep me on my toes, where if it's in a print encyclopedia, or an online one, I think there's a tendency to assume the book is right.
This may be a generational thing: I got to college in 1994, right when the 'Net was taking off for common/mainstream use: I knew from the beginning that many people were trying new things out, that not everything was accurate (or might be biased), and so on - where I'd been brought up with a deep reverence for books, especially more formal academic ones. Whatever the reason, I do find myself more inclined to read paying particular attention to bias and accuracy issues when I know it's *not* from a generally reputable source.
(In answer to the final question for this Thing, I did edit the 23 Things wiki, but I've obviously used others at various points.)