UPDATE: it has become way worse, now just a lame bait and switch front for a proprietary plugin. In stripping functionality out of the GPL plugin, you CANNOT trust this anymore -- any "updates" could easily be downgrades. Best to fork an old version, strip out the nagware, and be done with the lame bait and switch.
The anti-spam functionality is decent in this plugin, but the nagware is obnoxious -- register at the plugin author's site for a free licence key to "unlock" advanced features? Look at a sidebar full of advertisements for his other plugins and services?
This is not the standard I've come to expect from solid WordPress plugins on WordPress.org.
I've hacked the PHP to disable all the nags and unlock the features without a licence key. Strongly considering publishing the code as a fork, if the author insists on keeping the nagware in his version.