Integration of third party tools with Google Apps is nice, but having been burned using Sunrise and having had more than enough troubles using Gmail outside of the web app (such as via Outlook or Thunderbird) or native Android client the shine on Google Apps is a bit dull for me at this point. I'd be much happier working with services that use well-defined open standards that work with my choice of client software.

I know that Sunrise isn't a Google-apps specific software, but after: * a co-worker sent me a screenshot of a meeting invite I had accepted via their app (along with a facetious question "You get a new assistant? Who's 'Sunrise' and is she cute?") that makes Sunrise branding too prominent for it to be appropriate for a professional environment, and * finding out that instead of masking a response with my email address it had sent it from a generic "invites@sunrise.am"-style email but showing up as my name "Brer Lapn <invites@sunrise.am>", and having to then contact over a dozen colleagues who had been emailing the black hole address at Sunrise--- I'd say these services are less suitable for business purposes than the standards that Fastmail uses (and publicizes). Fastmail has never screwed around with my emails or calendar invites, and unlike Gmail I can use multiple different clients without it filling my drafts folder with autosaves.

TL;DR, if I had a say I'd be running our company tools on Fastmail rather than Google Apps (although deliverability is definitely something I'd consider a showstopper, I've never had an issue with FM).