Anyone successfully Use Approval by Email?

We can't for the life of us, get the Approval by email functionality working.  We have had very mixed results with this much-desired functionality.  We have had success when an email is NOT in the Approved Email list.  We have had success when the email IS in the Approved email list.  We have also seen approvals not go through in both cases as well (when the email is in the approved list and when the email is not in the approved list).  Would love to know if there is some magical setting to make this work.

Thanks,

Jason