In my opinion, the coolest thing is being able to use snippets and TVs (template variables) to create really,
really "idiot-proof" sites. For example, let’s say they want their homepage to display the latest news, blog posts, and something else. You can set the homepage up so they never have to touch it; they just create new content and it happens automatically. Example here:
http://simpleprayer.org
And when they create that content, you can specify exactly what you want them to enter. They want a thumbnail image next to each new article? Just make a TV for an image. When they create the article, they just click and upload their image. Everything happens automatically (after you’ve set it up that way, of course).
I have 2 clients that still call me for help after a year of using Mambo/Joomla. I don’t usually need to help my MODx clients after 1 or 2 weeks of them using the system.
Big recommendation from me though: Customize the FCK Editor toolbar to remove all the junk that they’ll never need. It’ll save you some headaches and keep their code clean. And teach them how (not) to paste from Word, because everybody does it, and it’s horrible