I have upgraded from Editor 2 to Studio 4 and it seems to be working well for me. I have spent quite a few hours on several projects and no problems so far.
There have been some issues with the way multiple track editing, in the one Melodyne window, is functioning (or has been advertised to work, I'm not sure now) inside Sonar. There are some other threads related to this. Certainly the 'stand alone' version has a slightly different feature set than the 'plugin' version at this point in time.
I also believe Sonar (and maybe Melodyne as well) are still working on the ARA functionality, and this will improve over time.
This is one of only a few 'plugins' that I use outside of the ones that are included with Sonar and I have been nothing but impressed with it since its release.