I clearly remember the moment of excitement and joy couple of years ago when I discovered drush. Interacting with drupal from the command line? Wow! This was definitely something new to me, never seen it before in any cms.
Digging more, I discovered the wp-cli for the wordpress, but it seems that this in no way as mature as drush.
I use drush on a daily basis. For simple tasks such as clearing the caches or updating a feature to more complex stuff like in a deployment process or as an interactive php shell.
As with drupal, drush can also be extended. It is just a matter of writing the proper hook in the proper file. It is really simple and everyone can write their own drush commands.
As an example, I am going to show you how you to trigger a node save.