Does usability suffer if you don’t follow the iOS Human Interface Guidelines?
When you don't use the iOS Human Interface Guidelines, usability will only suffer if your deviation doesn't work well.
Following established patterns is always a good idea, but if your product would benefit from deviating from the established patterns then it's worth doing. When you deviate it is not always going to result in a poor user experience, but the chances are higher, so you've got to test your new way of doing things on your actual audience to see if they find it to be a problem.
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