I just read this quote in .Net magazine:
…rather than [device-specific] breakpoints, I’d decrease browser width until things looked broken or cramped, add a breakpoint and repeat…
This is what I’ve always done and is exactly the way we should approach making sure responsive layouts behave properly.
I never thought the idea of having pre-defined breakpoints for iPhone, iPad and desktop etc was particularly useful, and in fact it seems much more aligned with an adaptive layout. Even when responsive design was just a baby there were already hundreds of devices with different widths so setting a few different breakpoints has never made sense.
At the end of the day it’s in the name: breakpoint. Find out where your design breaks, then fix it.