Zoom It is fantastic! Sure, its a one-trick pony, but what a trick! And it is far more usable than Apples built-in accessibility zoom feature. I love my Macbook Pros high resolution display, but all those tightly packed pixels mean that text can get pretty small sometimes. This is never a problem with Zoom It though. Also, since Zoom It provides a fully functional and resizable loupe centered around your mouse pointer, not only can you see whats inside the magnified area, but you can also work in there too (especially with the rectangular loupe)--typing, highlighting text to edit, gesture-scrolling, clicking on widgets, etc.
Zoom It also really shines on a Mac Mini (or any model) thats being used to drive a home media center. Its often hard to read a web page or the EyeTV program guide on the HDTV from the couch. But now I just Zoom It!