This is a great question because it really depends on what you mean by customizable. To most customizable would be integrations. Thankfully all of the major/enterprise level players have APIs that allow them to integrate with almost every CRM and most accounting packages out there. Real-Time analytical reporting is another area that is customizable when it comes to alerts/warnings etc. What is not as customizable in most cases (although some would want it be) is the name/labels of certain metrics both on phones and reports.