I've tried many forum software available on the market, and by far, xenForo has the best designed Forum Software on the market today. Not even Internet Brands' ownership of vBulletin has outshined it. In fact, it's a shame the king has fallen that hard.
I started with forums in 2001. I've tried iKonBoards (loved one of the skins, and I wanted to launch a site powered by it), then I discovered Burning Board, which powered one of the sites that I had minority ownership in, then I discovered phpbb and ended up not liking it, at all. I discovered vBulletin since everyone was using it. vB2 was just awesome, then vB3 came. Few years afterwards, I discovered a bunch of stupid free forums, then discovered IPB, never liked it. Then I discovered SMF, which is the most stupidest forum software I've ever witnessed or used. Just stupid. Not so simple, is it? SMF? IPB is right up there, but it does some things better than SMF or iKonBoards. I discovered PunBB, was eh, meh. Nothing special.
I've been attending ForumCon for a few years now, and I discovered Forums.net, Vanilla Forums, Discourse, and ProBoards. None of them has done anything like xenForo.
I gave vBulletin a chance to redeem themselves until the vB4.1.4 update. That was it. I fought with the IB staff over it, and then everything snowballed from there and they refunded me. vB5 while it looked nice on paper, was and still is coded horridly. I laugh at sites that use vB5 nowdays. They're vulnerable.
xenForo's only weakness is the moderation team. Sorry, but true. I've been a member of the community since 10-20 days after the launch, I even was one of the beta testers, always gave them honest feedback, suggestions, errors, etc. and the only thing they favor is the trolls.