Steve Jobs is often seen as a dictator handing out edicts about how users and developers should be doing things. These include rules about walled gardens, media formats, development languages, even how an application should feel when being used by someone. While some call him a tyrant, the last item is a stroke of genius. The Apple's Human Interface Guidelines, or "The HIG," is a key part of Apple's success.
The HIG blends usability and design
The HIG has one main goal—usability. It describes how an app should behave and look so that new users aren't confused, so that power users are not slowed down, and so that people with common disabilities can work with it. Imagine a world where individual developers choose their own copy/paste shortcuts, move the minimize button anywhere they want, and make the buttons on their app look like puppies. While programmers technically can do these things, they generally do not because users would complain about how hard the app is to use. A great app is a blend of usability and design. The HIG is there to help with that blending.
Why you should care
Users want powerful tools that are not only beautiful, but also easy to use. Initially, developers tend to focus on the "beautiful" part of the equation. When starting a project with a client, the word "sexy" is brought up a lot. The client wants the app to be stunning, but they may also want it to include the company colors and logos galore. Unfortunately, this often leads to "Photoshop syndrome"—the initial design drawn up by the marketing team is all a client can imagine—they ignore all suggestions to the contrary. That's where The HIG comes in.
Understand that The HIG is not in place to make all apps look the same or to prevent you from achieving your vision—it's there to help you make a successful app. The HIG was developed concurrently with iOS, and it embodies Apple's knowledge about what works in user interfaces. Remember that Apple is not out to sabotage your app; for iOS to be successful they need your apps to be successful. The HIG gives you plenty of room to maneuver—the guidelines are just to keep you from bounding over the cliff.
The design of the human interface makes or breaks most app projects. It's dangerous to stray too far from The HIG.
Read The HIG!
Before making your first iOS app, read The HIG! It tells you what the users of your app will expect and gives you insight into the platform. After reading The HIG you will have a better understanding of how you should design your custom controls. More importantly, you'll understand where you should not use custom controls. Standard controls leverage the strength of the underlying system. Take advantage of the fact that iOS users are already familiar with rotation, input, clickable areas, visual cues, scrollability, resizing, and so on. Having a deep understanding of these things will help you decide when an app really needs custom buttons or layout.
Sometimes the path to understanding is hard. I have seen many clients go through revision after revision of a screen only to wind up at back at the one that was suggested to them at the beginning of development. Eventually, most people see the light, and the time involved is well spent. An app that is well designed from the start will help prevent wasted man-hours and lead to less headaches during updates.
It isn't carved in stone
This isn't to say that you have to stick with out-of-the-box controls. Custom UI can add a lot to an app. Just keep in mind that an app isn't just screenshots, it is an experience—that's what keeps users coming back to your app over and over again!