Now I've started my new job I'm looking for a few little minor projects to do to help the transition to our new building and all the cool new stuff that will be inside it. This first of this is a wiki. I've never really set one up in anger before so I'm not sure which one to go for. I've found three so far:

MoinMoin
TikiWiki and
MediaWiki

I've installed TikiWiki on my box at home which seems to have a lot of configurability but it looks like it'll take a while to get going 'just so'.

Anybody got any recommendations/horror stories?

I am not currently wikiing anything, but if I were, I'd be using DokuWiki. I gave it a test run a while back and it met my needs above the others you mentioned (all of which I had used before.)

I've been using MediaWiki for quite a while, and I like it. I'm using it with memcached and it's plenty fast for me.