Coming from Google Apps and currently using Fastmail: It's not as good as Gmail. I still get spams in my Inbox but they are not really a deal breaker. I've been using Fastmail for over 3 months and I'm rather happy about it (just paid for a yearly subscription). I'd suggest you to try it out with a 3 month subscription. You can always switch back; it's just a couple MX records after all.
Oh btw, fastmail also has CalDav (Calendar only; no Tasks) and It's been working well for me too.
Oh btw, fastmail also has CalDav (Calendar only; no Tasks) and It's been working well for me too.