Anyone that votes less than a 4 for this great plugin has no idea of working with WP or has been smoking strange stuff.
I have been working with a lot of forums, such as bbpress, Simple:press, mingke, and others. But none of them had the functionality of phpBB. To install phpBB itself, you should indeed have some knowledge, but it is not that hard after all if you follow instructions.
But that is about phpBB.
The integration with WP is an other story, and this is why wp-united is so good: it's easy to install & use, it works perfect and I got GREAT support.
Anyone looking for a good forum should use phpBB and WP-united!