I've used plenty of them (and like them for tech centric documentation) but let's be honest; they never quite caught on in a business context.
This is most certainly not due to a shortage of implementations (wikimatrix lists 124, no less) nor due to a lack of user-friendlyness (markup-free WYSIWYG ui's are dime a dozen now, from pbwiki to google pages).
I would rather claim that there is simply a fundamental impedance mismatch between the wiki concept and the common workflow in most companies.
Like it or not (disclaimer: I certainly don't), most workflows are still dominated by Word and Excel documents. Those don't mix well with a wiki, in fact they don't mix at all.
Moreover once you enter this battle you'll find yourself not only competing with the friendly closet fileserver ("Just drop it on drive Z:, Sue") and E-Mail ("I'll mail you the new version in a minute") but also with the ilk of MS Sharepoint, Mindtouch and a truckload of other workflow solutions that more or less "work", but most importantly usually allow the users to stick with their beloved MS Office suite.
Anyways, to make a long story short; instead of beating a dead horse yet again, why not look at the ideas that have caught on in the meantime?
Those would be the whole 37signals stuff (basecamp), dropbox, and I guess you also have to count pivotal tracker into that bucket (as much as I despise it personally). What most of these seem to have in common is the "do only one thing and do it well" approach, as opposed to the old "one tool to rule them all" meme.
So, my humble opinion would be that there's still tons of low hanging fruit in the latter market. A wiki on the other hand... Well, perhaps impl #125 will finally part the users from their Excel. But tbh, I wouldn't bet on it. YMMV. ;-)