If you use phpBB and WordPress combination, WP-United is the way to go. I had tried this plugin in its early stages and it had several issues, and at that time I went with another plugin that was available.
However with my site re-design, I re-assessed current status of all possible options and was surprised to find WP-United in such a good shape. Easy to say, it works flawlessly.
And it has several very useful features which came in handy: Theme integration and User mapping (auto and manual). Plus if Theme integration does not work out of the box, there are several options, e.g. adjusting integration level (off / medium / max; medium worked for me). And there is always possibility to add some manual tweaks to CSS (reset.css) to ensure the finest look.
Thanks for this plugin and keeping up the good work/support!