Zoom It was one of my most-used utilities, but I uninstalled it some years ago after discovering the memory leak noticed by others. I found no workable substitute; OS X's built-in zoom options are embarrassingly cumbersome. For example, you have to go through three successive menus to change the Loupe size. and even then you must drag the corners. by hand.
Happily, the July 2017 update to Zoom It has fixed the memory leak. I've reinstalled. and found it's rock solid. It's amazing how stable and easy it is to write and click within a zoomed window . I'm not much for memorizing keyboard shortcuts, but those that I created for Zoom It have become second nature, so often do I use them.. I think Zoom It qualifies for the description "a great app".