If you press the button marked "More Reply Options" it will take you both to the full editor and a preview of what you've already written.
The button in the top left corner of the editor (called "Toggle editing mode") makes the editor text-only like the old one. But it also blanks out the other buttons, so if you want to use those instead of writing out code by hand you'll have to switch editing mode, press the button and then switch back. This is utterly daft. I've already brought it to RTP's attention, dunno if it can be fixed yet.