I use mail.app to keep a copy of all my gmail offline on my MB, and I use it for normal operations, like read, reply, .. draft mail.. I prefer it so I can have various mails open, include pictures the way I want, etc, without relying on the browser.
However, I usually swap to gmail when I search for something. The search is simply going 10 times faster on gmail than on my mail.app (Tiger).
Moreover, at work, my mail.app can't synchronize for whatever firewall reason. So I rely only on gmail during the day.