I may be not seeing issues as we spent a fair amount of time using the "report spam" and "not spam" buttons when we first signed up for Google Apps, so it's been trained to our use case. It works very well for us.
They also have a very clever greylisting approach -- it compares against senders in your address book as one point of data which I've never seen done before because usually the spam filters I've run don't have access to that data and no open source greylist daemons do it to my knowledge