SitePoint has an interview with James Atkinson, founder of phpBB, on open source and phpBB.
As I explore and use phpBB, I like it more and more. The main reason I like it is that there are tons of mods for it. For example, I was going to implement my own http referer tracking, but then I found out there is already a mod for it.
There is one thing though I don’t like about it. Even though it supports multiple style templates, basically only the subSilver template is ever used. If you make a mod to a template style, you have to make the changes to all the template styles. Not a good design. So, the majority of phpBB forums all look the same since it’s so hard to support multiple templates.