Pros:
It's easy to set up
It's more or less bug-free
It has a vast number of features

Cons:
It's not easy to add new features
It's not nearly as efficient as it could be
Everyone else is using it, so you don't look original

It'll probably be easier in the long run to roll my own. And more educational.

EDIT: I would use a generic forum for a small site, but a larger site like ours is going to get regular spam attacks and a heck of a lot more posts, and I need to be able to get more hands-on with the inner workings. I can't afford to take a week to fix an immediate problem.

EDIT: In Matt's defense, he did update WWWBoard to fix security holes, though the script still only works for single boards and still looks ugly. I had to rewrite like half the code to add multi-board support, a better design, more elaborate error messages, and checking against lists of banned words and IPs. It was my first introduction to Perl.