Most often I use TinyMCE, occasionally I also used FCKEditor - now under the name of CKEditor. I prefered TinyMCE because FKCKeditor was huge (lots of code) but I don't know how they compare nowadays. FCKEditor's advantage was that it had a free file and image manager plugin while TinyMCE's was paid (but you could use a 3rd party file manager or write your own).
Another disadvantage is that the handling of html formatting is still not perfect and sometimes something doesn't work and the user is stuck unless they know html and can fix the content source by hand - I mean sometimes many nested html tags accumulate over time after the text has been edited many times and it takes someone who knows the stuff to clean it.
Also, people who don't know html often don't know the concept of using <h1>, <h2>, ... tag, paragraphs, lists, style classes, etc. so they may find it hard to edit content properly without creating a mess after some time (once I had to spend time educating my client about how to use the editor). But overall it can pay because structuring complex pages so that the user edits everything in separate fields can be much more time consuming than just throwing a rich text editor at them and telling them how to use it.