This is a great piece of software that plugs a very serious hole in iCal where it can silently send invitations to people without your knowledge, especially for recurring events which have events in the past. When I upgraded to Snow Leopard, I noticed that a number of people were being flooded with iCal event changes when I made a change to a current event which had recurrences in the past (for some reason, iCal was retroactively updating the previous events and generating multiple invitations for these past events!).
iCal Reply Checker is a great way to stop this kind of nonsense, as well as giving more fine-grained control over how iCal can send invitations.
The only thing I think the developer should add (which would get 5 stars, IMHO) is to summarise the information where there are multiple invitations for a single event. For example, I have an event with 40+ participants and when I make a change with the preferences set to 'Do not send emails' and 'Display Notification', I have to dismiss 40+ dialogs. At the moment, I have 'Display Notification' unchecked so they are being silently discarded, but it would be nice to see when iCal is trying to send invitations in this situation.