In my opinion, the breakpoints have nothing to do with viewport widths of popularly available devices, be they iPads, iPhones, Samsung Galaxy, super-sized desktop monitors, or whatever... To me, the great strength of Responsive Design is that you do NOT design for
specific sets of device widths.
Instead, you define a minimum width and a maximum width, between which you want your site to CONTINUOUSLY look good and function well - at ANY device width. Your breakpoint locations should then be determined solely based on where your particular site design 'breaks', i.e where it needs a layout modification, in the continuum between your chosen min and max widths.
As Adam hinted at above (before he veered off and listed 'commonly used breakpoints'

, "
There are no preset breakpoints. Just place them where YOUR layout needs them."
Responsive Design is supposed to be somewhat 'future-proof' in the sense that the site should look good on ALL devices, including those that don't exist yet. Take for example that device that is released next week with the odd width of, say, 386px and which is an instant success and becomes the most popular gadget in the world...!