Man, I really needed this per monitor DPI stuff two years ago. I had a Vaio Z which had a full 1080p 13" screen, about 170DPI effectively, but I frequently used it on a regular 23" 1080p external monitor. The result was that I was constantly changing the DPI setting (which meant logging out and back in) based on what monitor I was primarily working on at any one time.
I'm really, really happy this support has finally been added to Windows. Props, guys!