Yes, we were running dotnetBB before but due to performance issues we had no choice but to move. We also have plans for the future of this site that were better suited for vB 3.x. This site is running on Windows Server 2003 using PHP ISAPI and MySQL. I could not be happier with vBulletin! I'm a long time user of it from early 2.x days and really like the new version. It's well worth the $$$.
Don't get me wrong, dotnetBB is a great system and I use it in another site. However, for a standalone bulletin board system, with more backend control, vB was a better choice. Unfortunately there are no importers so the old data will have to remain until vB comes up with one, which I doubt they will. We had to make a command decision early, although nearing 2,000 members on the old system, it was better then when this site crosses 20,000.
Their latest release 2.42 EC SP1 corrects the issues FYI. The folks at dotnetBB have been working hard to ensure top performance. It is a fantastic product for those looking for .NET forums, and by far my first choice for pure .NET forum'ware!