Pros: If you use Safari, or any other WebKit-enabled application, and like to live on the edge, then NightShift is for you! NightShift will automatically download and install the latest development copy of WebKit, but not before it creates a backup copy of your previous WebKit installation, just in case.
Cons: NightShift makes living on the edge easy, but nightly builds are not for everyone. Sure, some nightly builds offer new features and new bug fixes, but others offer plenty of new (and sometimes critical) bugs. This is, of course, to be expected. Bugs are part of the development process. If you want to live on the edge, you must be willing to accept the consequences.