How does this compare with Nylas (https://nylas.com), who claims they're the "next generation email platform"?
If I remember correctly they are building a base email application that can be extended with plugins (like the atom editor). This does require people to adopt their email client.
The InboxSDK takes a different approach in that it lets you extend Gmails functionality and write apps for Gmail. End users don't have to switch their email client (assuming their using the gmail web client)