Now you see, I've actually tried vb5 and really like it - The reason loads of people are complaining is because it's different. vB are going in a new direction, and its one that I think is needed, ofcourse there is going to be hate - No one likes "change" unless it actually improves things, and no one is giving it the chance it needs. Kudos to vB for sticking with their decision up to now however.
IPB is a good forum system - but it's done little to change it's clunky ways since releasing its current major version a few years back. IPB are mainly focused on their side products, IP.Content, IP.Nexus, etc. If you want to refresh the "whole site" or add systems on the side, IPB is the way to go. I think they know the forum industry is change, 3-5 years ago it was feasible for a site to only contain a forum, now, a forum is just an addition to an already functional site. IPB realise this, and have made products that work with the forum system rather than working for the forum system. It also explains why there have been no major updates to their forum system since 2012...
Xenforo, meh, I've never really liked it.. In my eyes its too lightweight (limiting), too fluid, the default skin is terrible as a standard (looks nice... but functionaly terrible) - They have good features, but the overall execution is poor in my eyes - It's like the older version of IPB. It appears the Skinners have decided to adopt this as well.. It's a shame. The design and features do nothing to engage it's audience, something that we have learnt is needed to compete with Facebook.