Yeah. Google does have great uptime, generally speaking. And it's true that it takes some years for an in house e-mail system to catch up. Of course, you have to get used to the idea that every word you send through anything Google is searched, indexed and stored. They sift all of it, and you sign an agreement when you sign up with them that this is OK. And of course, their service agreement is "best effort" so you can't fault them for any amount of downtime.
Would you hire a mail admin with that caveat? He'll make it work, as much as he can, but if he does not, you can't hold him responsible. He's cheap though, of course.
I don't know too many folks that would make that hire. So why use Google?
The answer is if you don't mind the search (making money off your IP potentially) or the cost (you're using it for less than three years or so, even for a MS compare) and you have execs that are comfy with the "blame Google" response when the thing breaks.
There are plenty of small businesses and folks that don't mind all that, but for the enterprise, it's a harder sell.