I recently took over a site which is mostly a phpBB version 2 forum for a niche audience, surrounded by a few fairly static pages written in PHP. Our forum user base is about 3000 users, with a hundred or so forum posts per month. We have modified a few of phpBB2's source code files from the standard issue.
Now phpBB version 2 is no longer supported as of mid-2009, though alternate phpBB2 support communities exist. The current version is phpBB version 3. The outgoing webmaster said that they had decided against upgrading to to phpBB3 software because the expected it to be a difficult task with little feature benefit.
What is the general experience with upgrading a phpBB2 site to phpBB3? Is the experience generally difficult, or generally easy? phpBB3 provides a conversion path for phpBB2 sites; how well does this work?
Let's leave the question about the benefits of upgrading to a separate discussion. Let's just talk about difficulty.