For several years I've been a big user of Google Apps. All my email routes to Google's servers and with the great IMAP functionality I've considered myself fortunate not to need to run email servers much anymore.
A few weeks ago I moved from using the powerful Apple Mail and MailActOn combination to using the Google web interface. While I still sync up with IMAP every day or so and will continue to use Mail both for offline access (though this seems to be coming along in Gmail) and some advanced features like integration with iLife and other apps, Gmail has a few features that Apple Mail can't match.
One of the big features is an addition to the labs tab in Gmail which provides a seemingly simple "Send & Archive" button.
Read more about Gmail as lifesaver