We have been using Cirrus Insight together with Marketo's Gmail plugin for over a year now. I don't recommend it (we are now shopping around for alternatives).
The chrome extensions seem to fight each other, with the "Send with Cirrus" and "Send with Marketo" buttons fighting for the correct spot.
(The "After Update" shows the buttons overlapping, with the "Send with Marketo" button mostly hidden behind the Cirrus button)
We only use Cirrus so our reps can access Salesforce templates from Gmail, but we actually send with Marketo. One of our reps noticed a problem today: normally typed emails work properly, but emails using the salesforce templates no longer have a subject in Marketo. It appears to still track open and clicks correctly, but we lose context of what the email was about.
Also, Cirrus isn't "really" integrated with Marketo. Instead, they allow for Salesforce custom fields to be shown through the Gmail sidebar. The Marketo-Salesforce app includes custom fields, which the Cirrus app can also access. So Cirrus and Marketo are integrated in the same way any custom fields you have in Salesforce (on Leads/Contacts) can also be "integrated" with Cirrus.
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