I am running the latest nightly, and the responsive back end is amazing- this is long overdue. I am developing on windows 8.1 and normally keep my browser at 1/2 screen, so I am using the collapsed menu's and so far everything is great. The design changes to the admin section thee is also great, and setting the theme to dark is very easy and better on the eyes when developing on small screens.
I am also really enjoying the theme page layout and responsiveness!
No complaints so far, just wanted to pass along the positive feedback.