If you're creating a brand new forum from scratch, and have the time to build your plugins incrementally (like avatars, PMs, etc) as your user list grows, this could work.
If you have a client that has had a phpBB2 since 2004 and wants to keep the years' worth of archived conversations, upgrade the forum software and improve the WordPress site, then this isn't going to work.
My solution for now is to keep a separate forum with a WordPress site and duplicate login info (for protected content in WP and in phpBB3). I tried dozens of times to import the large and involved phpbb3 database using the internal converter (including the current bbPress beta version) and it always hangs up "converting topics 100 - 199". Users got imported, some topic titles made it, but it never finished. And even with the repair tools the posts that did make the import were never organized correctly.
Another option worthy of exploring is WP-United. I've had better luck integrating the current forum with WP with that plugin. Now I just have to force it to function with WP eMember and we'll really be in business.
Hope springs eternal and I'll be watching to see how bbPress develops, but today is not the day to meet my client's upgrade needs.