Update: The 7.1 version due out in October is going to fix the issue altogether. Great job, Adobe!
Update: The 7.0.1 patch for Premiere Pro CC fixes some of the below mentioned issues, although unfortunately not all of them.
To my great chagrin Premiere Pro CC changed the way curves operate. Right now the curves, both RGB, and Luma, clip the superwhites and superblacks, and there is no interpolation going on after the curves hit 0 or the white level (255 or 1.0). In CS6, the curves followed the general slope, and it was possible to recover some of the “overshot” material. Right now, if you stick to curves, all clipped data is lost.
This is completely new, unexpected, and if you ever used curves, it changes your workflow dramatically, even if you don’t know it yet.
It means that you must remove the superwhites and superblacks from the clip before you use RGB curves. It means that if you were like me, using curves to apply the basic correction and contrast in one go, you cannot do it now. You have to first make the signal “legal” – reduce the superwhites and raise the superblacks with for example Fast Color Corrector, so that they fit between the range that curves operate on – RGB scale that is not overshot in either direction, even if you are working in the floating point (max bit depth).
It also means that there is no real backwards compatibility within the projects that used curves. Your colors will not be the same, if you had any superwhites in the project. I highly advise you to finish your current projects in CS6, and only then create the new ones in CC, being mindful about the necessity to use Fast Color Corrector before applying curves.
Adobe is aware of this issue, and hopefully some fix will come soon, but while using CC 7.0.0 version of Premiere you need to remember about this very real problem.