For all the talk about not re-inventing the wheel on these forums (I've even given this advice to others myself), I find myself contemplating doing exactly that.

What I want to do is to create forums for the members of a particular site where its really just an important extension of the core site purpose, which is online auctions.

Previously i've used phpBB2 on a subdomain to relative success, however the fact that phpBB is a standalone system and not designed to integrate well at all made it a bit disconnected from the main site. Not to mention constant spam.

I see sitepoint building their other sections around vBulletin, that is, I only ever signed up for these forums but I have been able to buy books and use the marketplace with my existing account.

Of course the big problem with using vbulletin or phpbb is that as soon as you start customising, it makes it way harder to upgrade, which is so vital at least from a security standpoint, given the fame of these systems.

I guess this leaves me with 2 questions:
a) has anyone had success integrating phpBB, vbulletin, or any other forum system with a foreign application to any success?

b) has anyone built their own forums to overlay a site, and was it hard?