A week ago, my server died. I raced up to the office, grabbed a spare MacBook Pro I was about to give to my new employee (so he had to wait a little bit, oh well), and in about an hour I'd transferred the whole system over to the laptop and had it all running again. Try that with Exchange!
Then this past Sunday, armed with a rebuilt server i was back up there again and transferred it all back up. Not a message was lost.