This extension takes only seconds to install. All you have to do ito get up and running is create a new menu item, supply the name of your wp.com blog and save. It starts pulling posts off wordpress.com and into joomla with no problem.
The only quibble is that the header for each post is active and looks like it should allow you to click through to the blogpost, but that isn't working for me yet. Clicking the header just takes you to a blank page on your joomla site (or at least it does on mine). But if you click on 'add comment' for a individual blogpost then it takes you through to the post on WP.com. I haven't yet got to the bottom of what's going on there.