I use it on my site directly from within flex. (not flexviewer though)
Like the link that Gady provided.
This allows you to log flex events as "pages" in google analytics.
eg Each time a user clicks on the print button, it is logged as a "page" in google analytics.
To use in flexviewer you would have to set the "tracker" as a global variable.
Then each event you want to log as a page, you would use tracker.trackPageview( "/PrintButton")
Where the argument is the page name logged in Analytics.
This is useful to find out what functionality is being used on our site, what is not being used.
Then you can ask questions like why is it not being used, hard to find, too many clicks, not user friendly etc.
Very useful tool. We have been using it for over 12 months.
See attached for an example of the output I get.