There is a nice article on Wikipedia Signpost about the experiences of two college classes that used Wikipedia as an assignment. The article is quite objective and informative, despite the fact that it is written by the professor of the class that had a successful experience with Wikipedia. That class (another article about it is here that includes some honest feedback from the students), a literature class, teamed up with a group of wikipedia regulars (the Featured Article team) and ended up having 3 articles featured on the Wikipedia front page. In contrast, in another class on global economics the professor had students upload their own Wikipedia articles, most of which were quickly flagged and deleted, which is more like a typical Wikipedia editing experience.
As the author advises, having students edit on Wikipedia requires some dedication on the part of the instructor, and most importantly, some experience with editing on Wikipedia yourself and experience with the Wikipedia culture and sophisticated ad hoc rules in place there (full of tags and acronyms). I personally would recommend having students contribute to Eduzendium (part of Citizendium) instead of Wikipedia, although Eduzendium hasn’t really had great success yet, either (at least according to another surely unbiased article on Wikipedia Signpost). But the participants on Citizendium are all required to use their real names. Therefore you see less vandals and AIFs (anonymous internet fwads*), and the wiki is smaller and the community more close-knit, which are plusses in my mind. Also, we’ll see if Google’s “Knols” project ever materializes as another viable alternative, as I wrote about previously.