Wikis are a neat collaboration tool. I've seen good ones, and bad ones. Largely, this isn't a measure of the technology itself (there are several different wiki implementations, but they all more or less function the same). The success or failure of a wiki purely depends on the people involved.
If you have people that are interested and dedicated to maintaining a particular wiki, that site will resist the general tend toward drivel which roams the 'net as surely as entropy stalks us in meatspace. If you have a neglected wiki, or a wiki in which the people do not cooperate in terms of organization/writing style/policy you end up with a chaos of random pages.
If you are going to run a library wiki on any topic and include public participation, I see one of two fates. Either it will suffer disuse - or administering it will become your entire job.
On the other hand, a closed membership Wiki for collaboration on a specific topic can be a very effective way of maintaining a knowledge base or discussion system. However, forum or blog software (anything with threading) tends to be superior when dealing with any sort of back and forth conversation. Having the ability to edit the past of a conversation is very disorienting, even if its possible to pull differing versions of the old page up.
The "hey, post something" attitude that is fine for forums and blog comments falls apart when it turns into "hey, read and understand our elaborate Wiki guidelines, please check quick to see if there is a more appropriate page, then post something and please have it fit within the tone and topic of the parent body as well as checking the Talk page for this particular topic to see if your change has already been discussed and rejected. Also, check back regularly to make sure nobody has deleted your contribution and replaced it with a Viagra ad".
Maintaining a good wiki is complicated greatly by the ease of editing.