...
A good rule has a very extreme S/O (near as possible to 1.0 or 0.0) and a high percentage of hits in the correct category. In other words, RCVD_IN_OPM_HTTP is a very good rule in the example above, because it hits 5.2028% of all spam mails without hitting any ham at all (no false positives).
S/O stands for "spam spam% / overalloverall%", in other words, the proportion of the total hits that were spam messages. As such, it is equivalent to Bayesian probability, or Positive Predictive Value in bioinformatics or medicine.
...