I'm a french user of ISPCP 3 and i have successfully integrated Postgrey with Postfix.
For me, the utility of Postgrey is to reduce ressource load of amavis when filtering spam.
I have a small server which received a lot of mail, and amavis make a high load on cpu, 70% constant with the spam filtering.
With Postgrey, i have reduced the load of cpu to 30% constant.
The conf was easy, just three config files, and if possible to integrate with ISPCP is to enable or disable postgrey per mail domain and per user, and perhaps give the user the ability to write blacklist and whitelist of sender.
I can confirm that Postgrey works like a charm with ISPCP 18.104.22.168.
See you soon !!!