My experience with Wikis are that are only religiously updated by 1-2 people. Everyone else just kind of goes meh. But you absolutely have to document things somewhere that is shared for the bus scenario.
My requirements for a wiki:
1) Awesome search. Xwiki has a very good search, including in attached documents
2) Fine grained access controls. I need to be able to have a space W with pages X,Y,Z, and Z needs to be public, but nothing else.
3) I need to able to use my existing authentication systems.
4) It needs to be user friendly. XWiki has some serious usability problems for anyone who hasn't used a wiki before. Confluence is a bit nicer.
My nice to haves:
* Site-based documentation. Sometimes I have a workflow that's 80% similar, but the exact steps depends on the site (Example: ssh to this host, run these commands). I don't feel that it justifies having a separate page for each site when the content is mostly the same.
* Editable from my Linux environment, preferably via command line
* OneNote integration
I've used mediawiki, xwiki, twiki, and confluence before.
If you can provide me with most of the things above, and either the source code, or a service SLA (or both), I'd probably buy.