I’ve been using Keyboard Maestro since at least 10.1 (Cougar).
Why do I like it? Primarily one reason: I can switch from open application to open application while hiding all the other open apps. Apple added Spaces to Leopard, which created virtual desktops for open apps. This was continued in Snow Leopard and changed to Mission Control in Lion. Spaces and Mission Control are welcome additions to OS X, but for someone who just doesn’t want the clutter of a million open windows, Keyboard Maestro is so much simpler and so much more elegant. Using Option-Tab, just a variation of CMD-Tab built-in, you can switch apps and never have to deal with the open windows of everything else running.
But, Keyboard Maestro is so much more. Did you use Quick Keys back in the OS 9 and earlier days? It’s still around and is $59.95. For $36, or less if you are upgrading, Keyboard Maestro lets you create macros to your hearts content. And, there’s also a place for saving clipboard items you use regularly.
Keyboard Maestro is one of the invaluable tools for my everyday use on my Mac. Wouldn’t live without it! You can find Keyboard Maestro here: http://bit.ly/Jqv6q0 or on the Mac App store.