I have come across the problem that if you use html in your page/post and click over to visual editor it strips out some of the html. I have disabled the visual editor for myself.
My concern is more for people in my network, who may occasionally use a bit of html, but don't usually. In my mind the visual editor "should" show a visual display of your html, not do weird things to it. I am almost certain this is going to cause a problem for some people.
I have read this is a known problem and that this plugin is useful in sorting out the problem: http://wordpress.org/extend/plugins/tinymce-advanced/
However, it was last supported in Dec 2009 and hasn't been updated to show compatibility for WP 3.0.1, even though some people say it works.
I don't really like using plugins that are not supported.
Does anyone have a recommendation or using this plugin?