I think a company wiki is only a good idea if there is some way to enforce style and organization. We use a wiki where I currently work (and my previous workplace), and they were both extremely useful and extremely frustrating.
Useful, because as others have already posted it is a quick and easy way to share information.
Frustrating, because if unorganized (and these were) finding anything is extremely difficult unless you have a direct link to it.
Here is a typical conversation in both of my jobs.
Me: Can you tell me where to find Topic X.
Other Developer: Yes, it is on the wiki?
Me: OK. Thanks. Can you send me the link?
Other Developer: It is on the wiki. Just search for it.
It is like me asking for my keys, and the response is that they are in the ocean.
Basically don't assume people will properly annotate and organize the wiki. There needs to be a person or team who ensures that all content on the wiki is organized and accessible. Without this organization the wiki can quickly grow cumbersome and useless filled with outdated, redundant, and hard to find information.