think its a great product thats free, easy to use, loads of skins.
i can navigate around easly and mostly not get any errors!
I preffer vbulletin hands down simpley because its more flexible from an admin point of view, easyer to add new features and subscriptions with ease.
the attachment function is simpley "great" you can view your attachments / upload / delete, what a great idea for a forum. i remember the old days of having to find a host to provide your pictures to the forum.
vbulletin may cost an arm & a leg but its worth it,
skinning vbulletin is slightly harder than phpbb.