I recently applied for (and was accepted) into Google's Gmail for your domain beta. I was fed up with keeping my spam defense systems up-to-date, always having to patch the mail software (postfix, amavisd, spamassassin, etc.), network outages, running out of disk space on the mail queue, etc. Mail is simply too critical an application for me, so I decided to let Google run it.
I finally got around to switching over my DNS mail records today, so now all of my email for prestonhunt.com is being handled by Google. So far, I'm really impressed with their service. I think it will be an obvious choice for small and medium sized businesses (and maybe even some large ones!) to outsource their email to Google -- it's a very capable system, with a slick administration panel, and (of course) it goes without saying that the service uptime, reliability, data backup, etc., will all be excellent. And with Google's engineers and servers running everything, a company would not need to waste valuable IT resources providing email service. Or paying for expensive Exchange and Outlook licenses.