After using this module for almost a year, and suspecting there was a problem for some time, I have realised that impressions on ads from anonymous drupal users are not always counted if there is any kind of cache turned on, this has totally screwed things up and lost me ad revenue, I confirmed this by haing a single ad (not rotating) on the front page of my site, in one day there would be at least several hundred pageviews at the absolute minimum, openx stats showed 10 impressions for the ad :|