Personally, I don't support paid software, I believe software should be open-source and available to all, to share; learn and improve both the software and themselves.
I'm familiar and experienced with most of the commonly used forum software, such as MyBB; XenForo and vBulletin, (I have used IPB before, however, I'm not much experienced with it as the others), now then, of course like many others, MyBB was the very first forum software I've played with, it's free and I was looking for something nooby not too 'professional' so I could work my way through and learn. I was amazed, MyBB was pretty neat, very easy to setup and use, extremely resourceful, and it got me thinking, why would MyBB be free when it has such great features, when other forum software that provide pretty much the same features be paid? The answer is pretty simply, MyBB was originally authored by a community guy, and he wanted to give back (very admirable), when the XenForo authors simply wanted to make money of their work (which I can respect, it's your work, you're entitled to wanting to gain from it).
I, however, find XenForo extremely neat, it's clean; organized; etc, but the fact I have to pay $140 for a XenForo license (and such for a license for similar forum software) so I could get features I could just get from MyBB is shameful. What's even more shameful, is the fact some individuals do admire XenForo, however, do not have the funds to purchase a license, and thus end up getting a nulled version of XenForo, which could get them sued in the long run by XenForo. Ugh, sad..
P.S: I have a forum that I run on a new open-source forum software called TangoBB, it's pretty amazing for a still-in development project, I'd suggest checking TangoBB out.