@ Aiden: generally, the ability to send leads to sales, and second, the ability to automatically get feedback on the disposition of those leads, really helps you get the most out of Marketo. If CapsuleCRM provides an API and you are on Marketo Standard or Enterprise Edition (not Spark), you can integrate the two systems.
@ Cécile: we have many customers with a bi-directional integration with a non-standard CRM system. It is more complex to implement than a one-way integration, and it will not be as sophisticated as the native integrations (SFDC & MS CRM). For example, Sales Insight is not available. But it is a full bi-directional sync.
@ Matt: if Opportunities are synced from CRM to Marketo, the full functionality of Revenue Cycle Analytics can be used. We have many customers doing this.