I would never be active on a forum running SMF or Yabb. I don't mind phpBB, although its limited features can be annoying at times. Like most people, I agree that vBulletin and Invision Power Board are superior to the others.
vBulletin stands out over IP.Board, though, because of better use of AJAX. The new version of Invision Power Board (2.2.x) looks more like MySpace than a forum with the whole new profile thing. I prefer the clean-ness of vBulletin.
Although on my forum, which is called The Fun Bucket (in my signature), I use MyBB. It's free, and has more features and a better interface than phpBB. I would really like to be able to use vBulletin (or even IP.Board in that matter) but the price tags for those bulletin board systems are very high.
I've been watching the whole vBulletin scene very closely lately though, and as a former Invision Power Board customer, the $80 (which was the old price for one year, they don't offer that anymore I don't think) I spent on Invision was a pure waste. Yes, it was better than phpBB and I could have more simplicity and functionality in my forum, but I just can't stand the company in general. Jelsoft, the creators of vBulletin, have great support and release patches and stuff that make it easier for the subscribers. Invision, on the other hand, enjoy to make setting up and upgrading IPB the hardest possible. As a customer, I noticed these things:
(1) The support is horrible. It takes a day or two to get a response, and it took me about 4-5 days before the problem was resolved. I had better luck just going on a forum about Invision Power Board and asking there.
(2) The plugin scheme is horrible. vBulletin has a great plugin feature, while Invision Power Board has a horrible one that makes you edit a million of the PHP files in the package to install modifications. I've also heard that the template system in vBulletin is much better than Invision Power Board's.
On an administration side, I've tested demos of vBulletin, and as being a former Invision customer, I was using IPB on my old forum for a long time, until my subscription ran out and I don't want to renew it. To be honest, I prefer Invision Power Board on an administration side with ease of use, ease of finding settings, etc. but that could be because I am more used to it. (The latest I've used is 2.1) vBulletin does have a nice admin panel though.
That's just my 2 cents...