Jumping to the breakpoint size would be useful to me – but mostly by way of checking the sizes directly before and after the break. I always like to check the edge cases. I don’t rely on common sizes for my MQs, but I will sometimes check them from the existing “responsive” dropdown list.
I like that chrome provides a visualization of the sizes, so I can see the breakpoints as I resize my responsive frame – but I wish that interacted the queries themselves. Tying that to the existing media sidebar, so Tim’s example would work in either direction, would be pretty handy.