Drupal Gardens has had WYSIWYG for 2 years now. We have quite a bit of data, both on upfront research in terms of technical expertise on which editor is the best choice, as well as on usability data.
I should have posted this data earlier, but just haven't made the time. Sorry. Better late than never. Here's the data...
Early on, when we were trying to figure out which editor to use, we compiled this doc...
After having CKeditor in DG for awhile, we hired a research firm to recruit users that had a need to contribute content online, but were not familiar with Drupal or Drupal Gardens. We also paired with existing Drupal Gardens users. We had lots of usability issues, you can read the full report here:
Summarizing, the issues were:
1. Users create content in Word, and paste into their blogging platform. Word to WYSIWYG experience is not what users expect
2. Input formats confuse users. They don't know what Safe, Full or Filtered means. We tried other options too with similar confusion
3. WYSIWYG is not WYSIWYG. After saving, they see the content in their theme, which is different than viewing via Seven admin interface or Text Area.
4. Clicking outside of WYSIWYG does not prompt for save. People loose data.
5. Going back and forth between WYSIWYG and HTML breaks formatting. Users expected the HTML formatting to be well structured.
6. Aligning images was problematic. People expected laying out a post to be easier
7. Working with tables was not easy.
8. WYSIWYG toolbar didn't appear to be editable
9. Expected block quotes to be a primary option
In addition to this data, we have customer issues in the form of tickets and forum posts. At the time of the redesign, one of the biggest customer complaints was the inability to add scripts, as well as have some representation of those scripts in the WYSIWYG editor.
Armed with that data, we created these designs, which have yet to be implemented, but are very high on our list of critical usability issues.