From what I experienced phpBB is a clumsy, narrow and very "flat" forum which gets hard to manage after a few hundred persons joined, as long as you don't install several hacks and then it will get ugly to install other hacks, and so on, and so on.
My favorite software (after testing lots of them, commercial and free) is myBBoard which offers you:
- Nice clean looking interface
- AJAX driven elements (for example quick reply)
- Mighty administration control panel
- Plugin support (no need for ugly hex)
- less mighty moderator control panel
- highly configurable user warn system via plugin(for version 1.2.x - guess it will be done for 1.4 soon *g*)