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I use Apple Mail exclusively. I only email lightly and I much prefer to have it to hand immediately as opposed to having to navigate websites.

If I was emailing a lot, I'd still use a mail application. I just find them much more convenient. I quite like Windows Mail as well (Not the version that comes with Vista, but the one you can download and can skin) for the ability to work with hotmail without having to pay for a premium account.
 
I use Apple Main to check my gmail account. It's pretty quick actually.. ;)
 
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