I went the opposite direction -- FastMail to Google Apps -- several years ago when FM had ~3 solid days of downtime. Really long time ago, but still a bit of a sore spot for me as I missed at least a day and a half of incoming email that wasn't deliverable during that time. My sense is that they're a much more mature company now though.
That said, I'm not sure why more people don't consider upgrading to Google Apps from free GMail. $50 a year gets you an SLA, support, and no ads. It's been extremely reliable for me and I've not had any downtime (that I've noticed) for 5+ years. No performance problems either that I hear folks complain about with free GMail either.
Obviously it would be better if there was no downtime, but the sending MTA should really have queued that mail and delivered it when Fastmail was back up.
Mostly because it doesn't defeat the issue with data collection. Google Apps doesn't, as far as I've ever been told, bar Google from using it's privacy-invasive "features" on your data. And while you can justify Gmail as a free service that violates your privacy to pay for it, Google Apps offers no such justification.
That said, I'm not sure why more people don't consider upgrading to Google Apps from free GMail. $50 a year gets you an SLA, support, and no ads. It's been extremely reliable for me and I've not had any downtime (that I've noticed) for 5+ years. No performance problems either that I hear folks complain about with free GMail either.