This document gets you started creating a CloudKit app that stores structured app and user data in iCloud. Using CloudKit, instances of your app—launched by different users on different devices—have access to the records stored in the app’s database.
Really nice to see Apple going to such lengths to get developers using CloudKit. This is a fantastic read for CloudKit novices and experts alike.
Apple has been upping their documentation game lately. Not only with new developer library documents like this one, but they now even have a (semi) regularly updated Swift blog.
I am quite bullish on CloudKit. I find much of it's API to be pragmatic and intuitive. I'd like to use it for some ideas I've had, but I'm waiting to see which developer or app gives CloudKit it's first "Letterpress moment". Not just to see how it handles the load, but to see how the usage limits shake out.