It was a breeze to install (drag it to the internet plugins folder and restart Firefox. Not only that but it is so much nicer to look at than Preview.
So, if you have a Mac and you look at PDFs, use this plugin. It is free unless you are a money making business, and even then it is only $69 US for a site wide license (as many computers as you can fit in a 5 mile radius).
UPDATE (25th October 2006) - This only works on Power PC Macs. Not Intel-based Macs. It won’t crash, it just won’t work. (Well, at least that’s what happened to me.)