I used to use phpbb, but when my forum got busy, my web host started to urge me to switch over to vbulletin, telling me it was less server intensive. Couple of months after that, my host insisted I make the switch and didn't give me any choice. Make the switch or find a new host. I made the switch because I like my host and never had a problem with them before that.

From the admin side, I honestly didn't see much difference in useful functions between phpbb and what was then vb3....But my members it turns out like vb better. They find it more user friendly and more comfortable, probably because that's the forum type they're most used to seeing.

All in all though, I don't see using phpbb or IPB instead of vb to be all that big of a deal from the user side of things. It's not like a lot of people will show up to your site, see an IPB forum for example and say "eew, it's an IPB forum, I'm outta here". To users, as long as it looks nice, and they know how to use it, it probably doesn't mean very much.

Ya gotta ask whether a new forum has less of a chance at growing if it's phpbb or IPB than it does if it's vb. If everything else is the same, I don't think it makes any real difference that's gonna make or break a new forum.