The long-short of Xenforo is this:
vBulletin's main developers wanted to rewrite the aging, PHP4 optimised codebase completely, to produce v4.0. (More or less following what IPS did with IP.Board).
Internet Brands, which own vBulletin, rejected their request with a 'we know best' attitude, and insisted on 'agile development'.
The vBulletin devs stepped down and formed their own company, producing XenForo.
Yeah, that's pretty much the tale. From what I have been hearing, despite the fact that its still in beta, I've seen a pretty positive response on many sites. And, yeah, I kinda noticed their site is a little bare, but
looking at their actual support forum paints a better picture. The system looks pretty swift, even on a forum as large as theirs (it looks they get quite a bit of traffic there).
And yes, they did make a vBulletin converter. According to their site, it costs $140, and it includes a year of free support and updates. In comparison, vB 4 is $190. That's actually not bad, given how much junk the new developers are trying to pile onto it, its only been almost a year since it was first revealed and they're already on 4.1!
And on the not enough documentation front, the devs put
a lot of videos
on Vimeo detailing features and behind the scenes stuff.
The style editor doesn't look like a big steaming pile (and I mean it, I actually ''did'' try out the vB 4 one, its indeed awful.)
Also, that thought of me being a bot is a little hilarious, given that I
did post an intro thread awhile back
