A while ago, I wrote:
On paper, this may be the most capable contender. It does diff and merge, provides scripting interface as well as a command line interface.
But then I concluded that opting for Changes may be a bad decision because the tool seemed abandoned and it may only be a question of time (if not already happening) that it becomes hard or even impossible to run Changes on a contemporary version of OS X.
Well, it seems as if right now the situation changes. As of today, Changes has a new owner who not only played nicely by lowering the price significantly but also released a compatibility update for Lion and Mountain Lion.
If Changes is really under active maintenance from now on this app is truly the first choice for anyone in need of a file (and folder) comparison (and merge) tool on OS X.