Apple’s smart battery case is good if you’re ever in a bind and need to quickly juice up your iPhone, particularly if you’re away from an outlet. Assuming the case itself is charged, I was able to get about an extra day of battery life from the iPhone XS Max, which already lasts me a day or longer.
It’s easy to use. You just slide your iPhone into the case and into the Lightning port built into the bottom of it. You can charge your iPhone and the case by plugging them into a wall together: the case is smart enough to charge your iPhone first before it begins filling its own tank. You can also just drop it on a wireless charger, which is convenient, but I found it takes longer to fully charge the case that way.
I recommend using a high power charger, like one included with you iPad, to fill both your iPhone and the case quickly.