I think that is probably a sensible way of doing it - it did seem a bit extreme that Letterpress is pure OpenGL. How much do you rely on QuartzCore/Core Animation rather than regular UIKit animation?
I think in an interview Loren said he wrote it in OpenGL for his own fun. Haven't we all done that? It's extreme, but it's very good at providing a lot of rich control, but with iOS7 a lot of fancy tricks he did will be pretty simple so I'd imagine that it was a one off.
I must admit I have done that, however I never really extended it very far. I think I was just trying to animate really complex bar charts really quickly.
I use QuartzCore layers extensively, and especially lots of Core Animation for the in-app animation stuff. For UI widgets like buttons I mostly made subclasses of UIKit components. So, lots of both.