Somewhat difficult, but everyone was extremely nice.
I recommend practicing code and a couple behavioral questions. I asked 2 questions both times and they were impressed like that as well. Explaining my thought process out loud seemed to go well.
The following metrics were computed from 28 interview experiences for the Apple Engineering role.
Apple's interview process for their Engineering roles is fairly selective, failing a large portion of engineers who go through it.
Candidates reported having very good feelings for Apple's Engineering interview process.