Ah, Wikis, who doesn't love doing a Google search and getting all of their information from Wikipedia?
I think Wikis are both good and bad. When used for groups such as friends planning a trip like we saw in the YouTube video or for Libraries, wikis are a great thing. When used for research, etc. I'm not so sure if I like them. I'm worried that the fact that anyone can edit a wiki might mean there's a lot of misinformation out there.
Another problem with wikis (and blogs, etc.) is keeping them updated in a timely fashion. I took a look through some of the wiki links and noticed that one said their book club ended in 2006 and they didn't do one in 2007. Um, we're now in 2010 so I'm not sure how current that wiki is.
In general I don't think they're a bad idea, but to me, it's just another thing you have to keep up with. If you're not willing to spend all of your free time maintaining it, why do it in the first place?