I just use these : 1024 (tablets) / 640 (mobile devices) / 400 (smaller viewport mobile devices)
You don’t even have to change everything on every breakpoint when you use responsive images and fluid layout, just what’s not working for you at that specific breakpoint.
Here is a list of devices out there in the world. Before you click this link (below), it should be said that breakpoints CAN’T have to do anything with devices at all. It is coming from design. So it’s not predictable at all anyway.
And to second Richard here:
“Responsive Design can’t be made without flexible widths and elements reacting on each other (inflow construction)”
Today (and using another app) I’m discussing each page-section and their rows entirely different from others. It would cause me having a lot of tabs in Freeway.