I have Tapatalk on all my forums... But with a good mobile skin/style, Tapatalk would become unnecessary for xenForo. I NEVER use Tapatalk. Call me old-fashioned, but for me browsing a forum is something I do with a browser, not an app.
Also I've found it a bit weird in regards to browsing long threads. It doesn't check what posts I've read in a thread, it just puts me on page 1.
I'm sure some people like/use it, but really, it could all be achieved with a mobile skin/style.
Look at m.youtube.com - it's WAY better than the Youtube App. Plus it gives you full control. You are not reliant on the makers of Tapatalk or the vetting process of Apple or Android. I mean, who knows what insane idea Steve Jobs is going to have next. First it's no boobs, then it's no flash. But it's okay to have an app that promises to make your genitals grow.