excellent plugin, which does exactly what it's supposed to do -
A few findings:
If you have a larger # of posts with no tags (>500), you need to increase your 'script timeout' in your php.ini file from (standard) 300 secs to evtl. 500+ secs.
Do not 'tag' a large # of posts with more than 5 keywords unless you can split them into batches of ~200.
If you don't need the plugin on a consistent base you should deactivate it. It will slow down your 'save post' and 'update post' cycles significantly.
Overall an excellent plugin, just needs some active supervision