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.