I've looked at several related post plugins and the thing that makes this one stand out for me is that although the plugin suggests related posts from other web sites, they don't make the selection, I do. That means I can make more relevant selections that look like they belong with the blog rather than distracting from it. They do make the default selection for related posts from within our blog, however, they again provide a quick graphical tool for changing the selection if I wish.
Zemanta staff went out of their way to be helpful when I had an unusual setup issue on our site. They could have easily stopped early on and told me the issue was on our site, not the plugin, but they stuck with me to solve the problem, providing rapid and knowledgeable responses.