Founder of the PaganWiki Project.
Version 1 of the project that I created ran on PMwiki 2b and was very successful. I chose PmWiki because it was easy to setup and maintain, and had a monobook type skin which would keep learning curves for other wiki editors down to a minimum. After about eight weeks, the time had come to get more functionality added to my project wiki.
On PMwiki, the volunteers do a great job of getting problems resolved and new functionality added, but it simply wasn't fast enough for me and I needed to jump ahead to meet my own project requirements. That's when I went to MediaWiki, which, as you may know, powers the [(approve links) edit diff] family of projects, including Wikipedia.
If you have the capability and need for a wiki engine that can run large wikis, check out MediaWiki, but if you need something easy to setup, don't have the capability or need of such a robust engine, than I recommend PMwiki all the way! For small scale or low/mid traffic wikis, there isn't anything better!