Interesting idea, but the following warning scares the hell out of me: "It can access: Your data on mail.google.com".
When you install a browser extension you need to trust the extension authors. In this case, rahulvohra's link below may help you decide :)
Please do checkout http://rapportive.com/privacy
Does that help address what was scaring you?
BTW. It would be great, if Chrome extensions would allow to add a link to the privacy statement which is then displayed in the context of the warning.