I downloaded and have been using Oak for a few weeks now and so far, I think I like it. I haven't seen ads and the app is pretty basic. They have guided and unguided sessions and I find myself returning to the guided version even though I've pretty much memorized the instruction. The voices are nice and I actually find myself doing a better job of staying focused on my breath.

I don't know that it's been beneficial yet, but it is becoming a habit.

It also introduced me to Alan Watts and that can't be a bad thing.