We also couldn't use any of the nice sales tools that integrate with Google Apps (or any of the tools that make managing calendars easier, like https://claralabs.com/ or http://boomerangcalendar.com/). We're now on Google Apps and very happy.
And also these tools get access to all of your email, which is a privacy concern. And in the context of a company, that can mean industrial espionage. Personally I cannot understand how people can trust things like IFTTT, but whatever.
I mentioned in a separate comment, Sunrise substituted a generic email address for mine in an invite response, but with my name ("Brer Lapn <invite@sunrise.am>"), which when someone replied to the group began to propagate across multiple co-workers' Outlook as the autofill address for my name. I missed email from some people for a week before one of my coders noticed it and let me know, at which point I had to get colleagues to forward me any message with that address, contact the sender and ask them to remove it from their Outlook autofill settings, and repeat ad nauseum until correspondence started coming to me again.
I sent a bug report, they responded "we'll look into it" and never heard another word about it, so for all I know it is still happening. If we'd have integrated that into the company processes rather than just my using it to consolidate my calendars it would have been a serious problem.
I'd love to know more about deliverability issues - that's something we monitor closely, and we want to know if there are any silent blocks/drop rules anywhere that we can't monitor for!
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).