Dave, it would seem the opposite is true of UBB. It's not well known because the UBB developers have a bad habit of banning vbulletin posts, and also any posts that have anything less than good to say about UBB. It's easy to get the impression that it's a well-supported piece of software, but anyone, [b]right now[/b], that knew about the long-standing (8 months?) security bugs in UBB, could destroy this board in an instant. They say that they won't fix the problems, but instead expect you to wait for Opentopic, their new "hosted" application. Scary.
vBulletin has released six releases this year, and just released a new point version to fix known bugs even though their 2.0 product will be in beta release in about a week or so. It uses PHP (faster than mod_perl, which UBB isn't compliant with anyway) and MySQL. There are UBBs out there that would die with 10K messages and 30 people reading, and there are vBulletin systems with 400 people reading 180K messages and still going. In fact, most of the vBulletin owners are UBB converts.
YMMV. I just think this board is a great idea and if I'm going to use it, it'd be a mistake not to tell you the problems you can avoid by moving from UBB.
I think those messages may shine a light on what I'm talking about. If you want information drop me a line, I'll help you out, no obligations. Just glad to have a resource.
Solo-1 Record holder, Mosport DDT