I have never used a wiki in my classes, but like Joel I have been experimenting with the Multilit wiki and I believe they have a great potential for EFL.
In fact, a couple of weeks ago I created my first wiki for a class that begins in March. I have made it private, but feel free to request me access. Do mention that you are a member of this EVO Group in your message. I would seriously appreciate your feedback.
Let me mention the main advantages of a wiki, in my view. It allows you to insert images, embed videos, create links, like a blog, though it is true the "visual" component is not so strong. Unlike blogs, you and your students can start new pages (and comment on existing ones, obviously), and place them in folders, which I believe can make for much more "organised" work - in a traditional sense of the work "organised", I admit. But I think (though I haven't tried it yet), students may find it easier to go to the "right" place and see what is expected from them.
I see the main advantage in connection with process writing. The whole process can be clearly traced (reversed if necessary) and collaborative documents can be produced.
I can't wait to implement this wiki.
I'm detailing a project in my blog. Perhaps you could give me some feedback?