[Chris8]

Depends on the person I guess. For me there is no such thing as brand loyalty. I simply chose what's best for me unless the price is really out of my range. As for the learning vBulletin I would be careful as imho the future of this software seems sadly uncertain for today. Personally I tied my largest online project with it and I put 4+ years in learning it and customizing it so there is no coming back for me. However, if I was starting to build my project today I would really carefully consider other options.

EDIT:
But if you asked me what is the best forum software you can get today I would say with no doubts that it's still good old vbulletin 3 series.
As for the vbulletin 4 series I see lots of improvement since it's beginnings in stability and performance but for me it still lacks too much even if there are some cool new additions builtin like activity stream.
As for vbulletin 5 ...I've never used it. From quick look at the demo/screenshots I think it's a completely different story.