Wikipedia is a great thing and has been my primary contact with wikis (the other has been the wikis ALA creates for conferences).
I have contributed some small corrections to Wikipedia articles (in one, the whole change was to change "non-profit" to "not-for-profit", but it was a very important distinction in that article). It's surprisingly easy to modify wikis, though there is something about the format (other than Wikipedia) that I don't see to be able to follow very well. But I think that has to do with the planning and design, not the basic idea of a wiki.
I kept thinking I had posted about this, but then I remembered that either on QL Chat or on another library forum I had posted something about using Wikipedia for reference questions. I looked through several library wikis and found them interesting. Many, though, I think could have easily been set up in other ways--that is, it felt like they were wikis just to be "with it". I would prefer to see the various versions and editors. Or a blog format would work just as well. For example, I think the ALA conference wikis could be set up as forums, like the QL Chat just as well as they could be wikis. There is no need for "correction" of posts and, in fact, I think that few are "corrected."
When I have done collaborative work, I've used a blog with comments or something like Google Docs. Maybe I just want more control than a wiki provides!