Update March 2016: the interface has improved somewhat since my last review (or at least the last review Apple recognizes, as it seems to have eaten my previous update). For some reason, however, the app turns off somewhere between 10 and 15 minutes which is very jarring. I may have to give up on this one, which is a shame, as I used to like it very much.
Update: Sorry, I have to downgrade my original 5-star review. The new interface has nothing to offer. That's the kindest thing I could think of to say about it. Would be nice to have a choice to use the old one.
Original review: I'm not always a fan of minimalism, but this app puts it to good use. There are no controls, only visual and audio cues to inhale, hold, and exhale. The opening tone of the inhale sequence changes subtly at 5-minute intervals so you have some idea of where you are in time. I'm grateful to find a meditation app that is free from cheesy music or narration. A gentle sequence of tones is all you hear.
It would be nice to have a control for tempo. A timer and reminder would be useful, too, but that's easy to take care of with other apps.