The price you pay for vBulletin is simple: it gives you support. If vBulletin breaks, you know for a fact that you can turn to their devteam for help, while with unpaid ones, you're stuck. killerguy, vBulletin doesn't suck, since you're paying for the support they give you. Even ipB wants you to pay for support.
phpBB 2.0.x is a very basic forum. It is efficient, but lacks many things the others do, which will be added in 2.2.x.
ipB is a close runner up with vBulletin, but if you want support, you need to pay for it.
I don't know, I like phpBB because there is such a huge community for it...... for example phpbbhacks.com - if you take the time to modify your board with all the hacks you want, you could essentially get all the features of other boards and more!
But I haven't used vbulletin in 2-3 years, and never used invision, so I don't know how recent versions compare
Based on my experience with the forum i am currently running. Invisionboard is an excellent powerfull free board. I recommend you to go for IPB.
1 issue remains un-explained for me.
Why are very popular and famours sites always using vb. WHT, Nuclearbox, gurn.net, etc and why not ib.
The only 3 sites that i have seen that big forum usage and online members are the ib site, ibplanet, and my own site.
I am going to restart my forums to get them re-organised and rid of the mods that make them slow in script generation times. I dont know whether i should be using vb or ib, but so far i am with ib, i am just waiting until the final release of 2.0 is out then i am re-doing my forums.