Very straightforward step-by-step instructions and it does what it says. I have a client whose 10-year-old phpBB forum could NOT be imported into any other system compatible with WordPress and dude wanted to keep EVERY. SINGLE. POST. WP United made it easy to integrate subscribers/paid users between the WP site and phpBB. Once it's working, set it and forget it.
A couple of caveats:
1) This integration is NOT for WordPress or PHP beginners. Not because of the plugin or because of WP, but because phpBB itself is not for beginners when it comes to adding MODs. You have to be able to follow directions in editing PHP code when using phpBB. There's no way around it.
2) This plugin works best with very sleek, minimal themes (if you're going for full theme integration.) The aforementioned client has a graphics-heavy theme with e-commerce, paid member management, and other special features out the wazzoo. So I turned off theme integration and it works beautifully to post elements of the phpBB forum in WordPress and integrate users.
Again, you have to know how to follow directions and edit code and upload individual files to make the plugin work in your system. If you're using phpBB, you should already have this skill.