A friend is running MediaWiki for his campaign, my campaign,and another friend's campaign. Spending some time up front to organize and lay out a few templates on how you want things to look has gone far towards making it work really well.
I've got a homebrew campaign world so something was really needed. We had a Y! group (that we still keep around for scheduling), but this really gives us a lot.
Me and one of my players are the most prolific -- I allow my players to help detail parts of the setting that impacts their characters that I haven't gone into depth. For instance, one player with a barbarian worshiped my my Native Pantheon (worshipped by Goblin kind, kobolds, and human savages) topok the gods that I had detailed lightly and went into depth about each, came up with some short myths and dogma, and just really made them live. This sort of thing, while he was doing it before the wiki, the wiki really helped come alive. As a DM with a homebrew world there are so many details, and having enthusistic players who want to flesh out points around their characters background is a wonderful thing.
Cheers,
=Blue(23)