Haven't used Xenforo, used vBulletin and IPB, absolutely hated IPB after I swithced from vBulletin and I really did give it a chance. Switched back to vBulletin after about a fortnight. IPB's admin interface is so awful.
I had a hard choice between ipb and xenforo, I have used all the forum scripts and do like ipb but currently using xenforo for http://scenemarket.org and it suits my needs perfectly with the xentrader mod and the pay to bump/pin mod. So i voted xenforo.