Our music practice app Soundproof has some cool custom user interface elements that we spent a lot of time getting right. The good people at Itty Bitty Apps interviewed me about this and how we use the excellent Reveal app to debug and design these elements.
You may find it interesting to find out a bit about how we work, and how Reveal is worth every penny if you’re an iOS developer. Buy it now if you haven’t already. It does stuff Xcode can only dream of.