This is great! I always wondered why this TinyMCE editor in WordPress is so poor, but when you go to their web site you see many many useful buttons. Finally there is a simple way to extend the default toolbar.
The thing I probably missed most was the U button. In fact I use to render underlined text with a maroon color by means of the WordPress stylesheet. I always had to add the markup manually, and in WordPress 2.1 it got trickier because I had to make a Visual-Code-Visual roundtrip, due to the escaping facility added to the Visual side.