Most of them are very happy with it. The system comes with a control panel so they are able to see at a glance e-mail which has been quarantined and release it to the queue if they need to do so. They can also set black/white lists and define a list of valid e-mail addresses for thier domain which increases the accuracy further again. The spam identification is very accurate, I have yet to receive any complaints about valid e-mail being blocked. If you want to have a look through the control panel send me a PM and I will set you up with an account and you can have a look around.
(c)David Overton 2006-23