Let's be honest, lacking behind a few months, especially if it is a major release, isn't a bad idea. With Rails apps in production I keep upto a year of distance to the major releases because there's so many rubygems that need to catch up or be replaced.

That's probably true. I think it took mLab a couple of months to support 3.6. But then again, having a major version release available at launch doesn't seem that important to me compared to all other database-as-a-service features.

Since you mentioned you work for MongoDB, if you guys could partner with Heroku and add Atlas to their official add-ons our team might be able to take a look and switch ;)