Best of all is the rapid support if anything goes wrong (applies only to commerical, pro version). I had some minor trouble than was solved with a new release to the plugin within 24 hours). Well done.
Here is where extended menu is let down. The documentation is there but not well organised. If you are in the same position I was in and need this running, and running fast, follow these steps.
1. Make sure your main menu is complete, with your entire heirarchy filled out.
2. Download and install this module.
3. Open the module in Joomla! and ensure the setting 'Menu Name' is set to main menu.
4. Publish the extended menu
5. Ensure it is positioned in your template, and that the corresponding 'Position' is set in the extended menu module.
6. Go to this web page: http://de.siteof.de/extended-menu-parameters.html and work through the parameters, trying them until you get the desired effect.
7. Go to this page: http://de.siteof.de/extended-menu-templates.html if you want to do dynamic menus.
It is unfair to expect the developer to produce a product that can be used by people without very good knowledge of XHTML and CSS, as this is not Dreamweaver. If you are not in this position, you really ought to pass this over to the person in your company who is - once the initial work is done, any of your customers (internal and external) need not look at this again and simply worry about adding content. I consider Extended Menu as set and forget at this level.
No amount of silly comments (as below) will help such fantastic open source products as Joomla! compete. Leave these for the trolls in the newsgroups.