I usually give clients the choice between Markdown and a WYSIWYG editor.
Editors are getting better, but there’s still a lot unwanted markup, breaks and spaces that gets introduced into the markup.
I’d say about 1 out of 3 clients can’t live without WYSIWYG, but I must say most people take to Markdown really well, once they get over the initial learning curve.
The selling point is always: copying a piece of text from another document into the textarea and presto: paragraph formatting is done! Most users only need to add a few links, some titles, maybe a bit of bold and thats it.
For some Mac users I’ve even installed Textmate (which has a bundle for Markdown) on their Macs to make it even easier.