I have used the poormanscron 4.7.x-1.0 module on production Drupal sites. I am using it for the CZEN.org website without issue. But....
In testing the poormanscron 5.x-1.1 module on Drupal 5, I have noticed some issues. It kicks off multiple cron runs simultaneously. In combination with the Notify module, this generates multiple emails sent. Poo!
Have you tested the poormanscron module on Drupal 5? Are you seeing similar?