I set up a wiki, again as part of the Blue 2.0 activities. Right now I don't think I have any need for it, though.
I do already have a wiki I created for Webmaster Section of SLA's IT Division. I've got to say that I prefer the default look of PBwiki to the look of SLA's Confluence wikis by quite a bit. PBwiki is clean and uncluttered while Confluence pushes a lot of options on me that I don't really need and the flow between pages kind of sucks.
In general, wikis are nice when you want many people to be able to edit a single web page. They seem to be quite useful for times when you have an active group of users that all want to contribute to information on a topic or just the general creation of a site. I have seen them used when I think a regular web page is more appropriate. Sometimes people are using wikis just to say they used them, which I think is a little silly.
So PBwiki is probably missing a lot of useful advanced options (why else would there be pay versions available?), but it only seems to be missing stuff that I don't need. Too bad I don't have any use for it right now, hence my wiki's name: Unnecessary Wiki :o(