I thought long and hard about moving from phpBB to vBulletin. And I did it for both tactical and strategic reasons.
Tactical: There are hundreds of forums out there, many of them (like phpBB) free or low-cost. However, they are not updated all that often with bug-fixes and security patches. vBulletin is definitely very well-supported and new features and functionality are added regularly. With phpBB and other forums, you have to add "mods" which often can "break" the board during installation, or have to be re-modded after a new software revision of the board is released. And even then, often with new releases, mods for older versions don't work. With vBulletin, there is a ton of features/functionality included (so not as much modding is required).
Strategic: vBulletin is not only a forum but really a fully functional content management system (CMS) if you want to license/add those modules. There is definitely a ton of headroom to grow and/or add functionality if this site grows. Also, the 500Ecstasy site used vBulletin, and one key reason I changed over to vBulletin was because if there was ever a decision to merge the old 500Ecstasy site with this one, it would be MUCH easier technically to merge forums that are based on the same platform. I have had some discussions (earlier this year) with Scott and Benny about this, and although there are no plans nor active discussions underway to do so, if we ever decided to do it, having this site on vBulletin would pave the way for a much more seamless integration. It would be my goal someday to have the sites combined as that site's resources are really fantastic (although this site is growing nicely and is accelerating, with much of the key content already here).
Hope this helps explain things a bit.
Cheers,
Gerry