I've got a Drupal 6 site and YUI working together just fine. There's two drupal modules that handles the integration at:
http://drupal.org/project/yui
http://drupal.org/project/yui_editor
I've only used YUI for the rich-text editor, not anything else, but the editor is great. All the usual formatting, plus the image handling is very cool.
The only thing I found tricky was getting it to work with a local copy of YUI. You have the option of using hosted-YUI or local-YUI. With hosted-YUI, you simply give the URL where Yahoo is hosting it. With local-YUI, you install YUI on your server. For my site, on my company intranet, I wanted a local version so we weren't always reaching out to Yahoo. You have to specify the filepath to where your local version of YUI is located exactly right or it doesn't work. And the filepath, from memory, was a little counter-intuitive (or un-drupal). Unfortunately I can't access the site from here to remind myself of the exact problem.
In short, go for it.