Lauren Goode lists all of the features — and there a few of them — of the Smart Battery Case, and summarises:
Apple’s smart battery case is good, then, if you want a softer case or a “passive” battery charging experience, with zero control over or understanding of how the case actually charges your phone. Maybe that’s what Apple is hoping: that buyers of this thing will slip it on and never take it off, charging their iPhones entirely through the case’s Lightning port going forward, forgetting about its big ol’ bump in the back. They will be pleased, finally, with their iPhone 6’s or 6S’s battery life, and the memory of spending an extra $99 for it, rather than having it just work that way in the first place, will eventually fade away.
For the record: I just wish the 6S had the battery life of the 6S Plus.