I've studiously avoided phpBB3 until now, as the gestation period has been extremely long and seemingly neverending (phpBB2 was released six years ago!). I've set up a test forum with the RC1, and the version 3 is a quantum leap over the earlier version.
My first impressions: the basic phpBB3 package has nearly 1000 files, it's a huge project! It is loaded with features, many of the most important mods are now an integral part of the main script: specific robots handling so Google etc. don't get session IDs, moderator and admin action logs, extra bbcode for Flash, Jabber, etc. The number of options for forum permissions is huge, there are no categories as such, and a forum can be a category with sub-forums. The new prosilver default theme is clean, modern, but due to the complexity of the package the overall look is rather crowded and confusing for the uninitiated.
I'm certainly not planning on moving to phpBB3 just yet, I'm not tempted to be an early adopter and I'd be happier after a few months of shakedown by others to spot any major bugs and security issues. But I will be upgrading eventually, despite the amount of work that will be required to simplify the interface down to something more user-friendly.
Overall, it's a very impressive package - on a par if not better than vBulletin, extremely powerful, and supremely adaptable.
Does anyone else have any experience or has anyone used phpbBB3 in a live setting yet?