Tapa talk is already 3 bucks, think the df will have to be free unless it adds its own version of tapatalk.
It's free to setup your forums. I did it for my personal forum I run and it was maybe 2 minutes of work to configure. That was with phpBB bulletin, vBulletin (this site) may be different, more difficult.
Ok before I didn't really think we needed one since the mobile site is "usable". but the more and more I try to view the forums on my droid the more I see how "bare" the mobile version is which compels me to use the real version instead. Unfortunately the real web version doesn't really function too well with browsers as editing is a PIA, quoting and other stuff I try to do gets really cumbersome.
So I'm for an app, or at least make some improvements with the mobile version.