VBulletin basically does the job of a forum software, but leaves issue undetected at first, that should be revealed before purchasing. Most of the issue cannot be seen by the general public, as they are hidden in the password protected user area. None of their software will validate via the W3, which to me, clearly means that the programmers are not concerned with testing their software to make sure it is written to standards and does not contain basic underlying errors.
Much much worse, is that the forum produces page after page of duplicate content, because every page can be addressed in many way. Your Google webmaster tools account will very shortly be filled with duplicate Title tags and Duplicate Descriptions and their is literally nothing you can do about it. VBulletin's response was to create a new version 5, instead of fixing the issues with version 4 first. To there credit, they do seem to be continuing to work on V4. But lets face it, if they cannot get VB4 to validate, why would anyone think they can do it with V5.
The support team seems to be working hard to try and help, but they do not appear to be supported by the programmers, so any issue that has any type of complexity to it, will not be solved. At least that has been my experience. Judging from the post of others, if you push for a resolution, VBulletin staff tends to block you from further posting. Not a very honest or open way to do business.
If VBulletin were cheap software, I would not complain to much, but it is quit expensive, so you would think it would be very well written as such. Unfortunately it is programmed more like cheap software, with obvious and seriously written flaws and a programming team that does not seem to be very professional.
I cannot recommend VBulletin, nor can I advise against it, since I cannot recommend any other software that might be better, although I would think there would be. Should you decide to purchase, be aware of the duplicate content issues, that you will face. I would run any of there forum pages through the W3 validator, so you can get a feel for the quality or lack there of, also