Wordpress kind of sets the standard for great interfaces for end-users. Drupal is a little more mixed: it is a great experience if the developer updates the UX when they update the site functionality. Concrete5 and other CMSs basically exist soley on the merit of their end-user experience.
Where does ExpressionEngine fall into the mix? How much control does a non-developer admin have with EE, and how pleasant is that interaction for them?
That's really up to you. The backend of EE is completely customizable. It's a blank canvas by default, so you get to build your own backend that is fully customized to fit the exact needs of the client. You can give them more or less control as needed, and you can create different user groups that see different things (including different modules, different menu items, different channels, even different fields within a channel, etc.)
If I'm working with a client that needs a fairly straightforward site (read: brochure site/portfolio site with or w/o blog) and they're not terribly computer savvy, I almost always go with EE because I know I can make it dead simple for them to update their content. For users with a little more knowledge, I tend to give a little more flexibility. It's really up to you how you set it up for them.
I've never had a client complain that EE was difficult to use, and most are actually really surprised at how easy (and some use the word "fun") it actually is.. great confidence boost for people who have always had difficulty updating their site in the past or had to rely on others to do it for them.
Hope that helps :)