Many people (far too many to attempt to list) have raved about the successes they have had
using Greylisting. Some have even gone so far as to put up webpages with statistics and graphs.
Here, I'll post links to external pages that people have created that contain real data, not just kudos.
Grey Mack has a great page showing before and after graphs with some statistical analysis:
Alun Jones has a graph showing the performance of his greylisting experience: