This is a very nice walk through Sarah of what can be done with the Customizer. Damian is right: eventually the Customizer will be an amazing feature. From what I understand, Menu Customizer is a bit more controverial though. Wordpress.org would have less issues if there were a dev version and a stable version. The stable version could be for businesses while the dev version could go out to people who enjoy playing with their websites and playing with technology.