I mostly experienced it in the 1.x versions, so my experience is coming from that.

As an administrator I appreciated that it was easy to setup. The control panel was very powerful, and it had a very good selection of themes and add-ons. I didn't like the cost ($150/yr iirc), but it was worth it since other offerings weren't as good in my opinion. XenPorta made it simple to make an attractive blog-like homepage. XenForo's templating system let me easily customize views and CSS styling. It had a simple like system, but there were some reaction add-ons that worked very well on the forums I owned/administered.

As a user I think it has a very good UX. The editor was nice and easy to use. Animations felt polished and unobtrusive. The email templates and notification systems were very useful as a user for receiving updates. It used modals sparingly and when it did it made the UX better.

I think 2.x is a lot more polished now, although I haven't used it much. I felt that 1.x required a few add-ons and a custom theme to be fully-featured, but with those add-ons it couldn't be beat. I think even modern forum systems could learn a lot from XenForo, but my opinion is of course biased since I'm so familiar with XenForo.