Whenever I have tried to use an Iphone I get annoyed that they don't seem to like the concept of a "Back" button in the same way that I am used to from Android.
I can't remember exactly but something like: I am in Contacts...click an email...it goes into email, I send the mail, click Back -- should be able to get back to whence I came, right, which was contacts? Nope, I am stuck in email and the only way out is to go to the home screen and find contacts again.
Or I am in any app, finished, want to back out and the app should disappear...nope, I have to jump to home screen.
So the famed "usability" is just what you're used to. Sadly I come across apps even in Android where the native "back" button is ignored and you can only navigate via buttons provided by the app author. I assume these are ported from ios apps.