Wikipedia - like any other lazy student I love it. The idea of pooling the knowledge of millions of people to continually build a collective understanding of a particular topic or question is an incredible one. No doubt there is a variety of quality that comes from the amount of participation and the quality of the discussions taking place. It seems quite often that the more obscure the topic, the more questionable the reliability.
These are some of my observations of Wikipedia - make of them what you will:
They tend to be a bit of a "balanced" overview. Areas of controversy often seem to be given roughly equal space, often when they don't need to be. They also tend to read like a composite of ideas (as of course they are) rather than pursue a single idea through a document, which make them a tough read for me. Maybe there are wikis that follow more of a pursuasive structure but I haven't come across any in my fairly limited exploring.
While not that new in internet terms, in real world terms Wikipedeia is pretty new. I feel like in some respects our culture hasn't really caught up yet - we're not sure how much faith to put in them. Teachers sometimes treat them with suspicion and tell their students not to use them - I doubt this helps our students develop critical literacy around these potentially rich sources of information. I wonder if many teachers have necessarily had the time to develop these literacies for themselves. I'm not sure I have.
As far as teaching goes, I think these provide amazing opportunities for collaborative learning. I'd imagine a good way to go would be to have small groups working each on a particular aspect of a larger topic - kind of like a "jigsaw" lesson. Students become experts in a particular aspect and collaborate with the other "experts" to shape their part of the larger document. I think a looser, more anything goes, approaches could work for more experienced students given a longer timeframe. The potential to collaborate between schools, or countries, opens up doors for cross-cultural learning and understanding, which is very exciting.