We've actually got SharePoint at my day job running as our public facing internet/extranet site (a bit of a hack) but also use a separate installation for the company intranet.
SharePoint on the intranet is mainly used by the different teams to create their own areas, calendars, documentation repositories, etc. We've got 1200+ people and several companies working under one big virtual company umbrella. SPS 2003 is nice especially if you are using Office 2003. Integration with InfoPath is really slick if you are into capturing information from forms.
We then have a Documentum installation that is used for storing CMMI required documentation. It is web-based and has version control/history but is annoying to use. Very hierarchical and a little slow to navigate through.
Now, there is an effort to bring all the technical documentation up to date (system has been around 20+ years) and the initial proposal was to create Word + Visio documents and store them in the Documentum repository. This essentially equated to shelf-ware that would never get used. SharePoint was suggested but even it has some obstacles to being a really good documentation tool.
I proposed using a wiki for the documentation and got MediaWiki up and running on Apache + MySQL + PHP in a day or so having very limited experience with any of them. Everybody has so far bought into the concept of wiki, but we still have another layer or two of management to get through before we can officially begin. We do have nice little proof-of-concept site going though.
I like MediaWiki because it is mature, runs WikiPedia, actively maintained and enhanced and has lots of really nice features like History and Talk pages. It took a little convincing to go the non-Microsoft route, but the only real ASP.Net option is FlexWiki but it just isn't quite there yet (although I do use it in my after-hours endeavours).
Matt