I have used wikis for some time now (not for any type of academic work, mind you ;)). At first I used wikipedia as a user only, and to the present, I still do. It wasn't until much later, when a radio program featured these new internet tool and explained how it worked that I understood the way a wiki is made and built upon. I have not contributed to any wiki or edited any existing one because I, personally, do not feel I know enough about any particular topic to pose in the internet as an expert or authority on it. I went and played for a while with the wiki sandbox, which allowed me to get a better feel of how a wiki works without risking an embarassing moment of posting an erroneous opinion without base.
I think wikipedia, and I suspect that other wikis do, too, suffer from a bad reputation from people in general. Time and time again we are told at our classes that we should stay away from wikipedia when doing our research. Many people are of the opinion that wikipedia is nothing more than the postings of people who don't have the credentials to post any kind of information about a topic. Many people believe that only those who cannot publish information credibly will resort to wikipedia to make their opinion heard. Neverhteless, every time I have had to check something on a wiki, their information seems sound and well researched.
So what is it? Is wikipedia a depository of common, legitimate knowledge? Is it the collection of opinions from wannabe experts? In my opinion, wikipedia is a good source to find information about mundane topics, but it is correct to stay away from it when doing serious research.