If you have never used Screenflow before, you will probably think IShowU or other similar screencast apps as suffice in their functions and performance. But once you have tried Screenflow, the choice becomes apparent instantly.
What Screenflow has that other don't is the clips/timeline editor (akin to iMovie) that it also incorporated. You are able to edit your clips in the editor, and compile it straight away. It has other unique callout functions that other mac screencast apps lack etc. Just download the trial version and give it a go yourself, or go to their website and see the demo videos to judge for yourself.