this plugin was working for me until I changed my PHP5 environment to include eaccelerator, and change PHP handler to FCGI (I'm on a VPS). I got this error:
Fatal error: Cannot access protected property efficientRelatedPosts::$_hook in /home/accountname/public_html/wp-content/plugins/efficient-related-posts/efficient-related-posts.php on line 30
I have had to disable this addon, sadly, since I love it and recommend it.
Would hope there's a bugfix possible.