XenForo is responsive, so it doesn't need an app. The 2.1 version (just released stable yesterday) is more mobile friendly than 1.5 (this version on Head-Fi), and even adds notifications for Chrome browsers (and a couple of others), including on mobile devices. I used to use Tapatalk on a few forums I run, but it was a security nightmare, had poorly written code and even worse support.
There are always rumblings of someone making an "app" but it never gains traction, since so few see a need for it. I don't like making users download anything to visit our forums; but, there is a simple line of code the admins can add that will let users save the forum to their home screen, which then in turn will open in a "chromeless" browser window that looks and sort of operates like a separate app.
I don't visit here often, but just happened to check in today and noticed the change to XenForo. Nice job from all those involved!