Price-wise, the cost is highly competitive. I have no issue renewing the license when it comes due.
But feature- and support-wise, it is a few steps ahead (if not more) of anything else I have looked at. I come from using some of the free forum software packages like phpBB and MyBB and found support to be uneven or nonexistent, release schedules erratic, and features that are added to catch up with the paid offerings out there.
Sure, like any software, I wish there were features in XF that I might be able to get elsewhere. But overall, it's a well-thought-out system that I've found easy enough to work with for customizations, and it can easily run both small and large forums without taxing the server like the system we upgraded from almost a decade ago.