I kept forgetting to check updates because the small site I help run rarely receives new content. As a result, we got unwelcome visitors.
Mail On Update solves the problem beautifully. Instead of me having to remember to check for updates on a regular basis, I now get a notification when my attention is needed.
In my opinion this functionality should be part of Wordpress core. It's odd that WP offers email notifications when people post comments, but not for something as important as updates.
From what I can tell, MOU does not check for theme or core updates. They are also important, so it would be nice if they could be checked as well.