1) Yes...to download at least. You need to have some form of Marketo Sales Insight (MSI) in order to invite new users to use it after it's downloaded (they need a registration code that comes from Marketo).
2) No, it only logs "Sent Sales Email" or "Receive Sales Email" activities to Marketo. You could, of course, use those activities to change some lead fields that could be pushed to SFDC. Not sure what you mean by the activity part. The sales email-specific activities that occur in Gmail are logged against leads in your Marketo Lead Database.
3) It's very similar to Yesware, but more focused and integrated into what's happening in your Marketo instance. There are also some Account Based Marketing features in the plugin. Example: If I am sending an email to tony@acme.com, I can see all types of Marketo activities that have been logged for Tony (Filled out Form, Visited Web Page, Opened Marketing Email, Interesting Moment Logged, etc.). I can also see what recent activities have been logged for Tony's colleagues at Acme.com. Perhaps someone had already sent john@acme.com a sales email yesterday, I can sync up with the sales person who sent it to see how that went before emailing Tony. Plus a bunch of other cool features. I recommend just trying it out if you have seats available for the Email Add-in under Admin > Integration > Sales Insight
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