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.
It's expensive for what amounts to a clunky way to extend your iPhone's battery life. There are other companies, such as Anker, that sell good battery packs you can keep in your bag. These external packs are cheaper, hold more power and can recharge your iPhone battery multiple times. Anker sells a $50 pack that holds more than 10 charges. But it's not as convenient as a case that's always on your phone.
Apple's case feels heavy, almost comically so, and I'm often very forgiving when it comes to oversized phones. You might end up wanting to leave it in your bag, but that takes away the point.
And — this is pretty obvious — but it's not water-resistant. So if you like that your new iPhone XS can be dropped in a pool and survive, that's not an option when you put on the case.
Should you buy it?
Sure, if you value convenience and reliability over all else. I really like knowing that I won't run out of battery and that it works with wireless chargers. It's great if you're going to get on a plane and don't want to fumble around looking for your battery pack and a cable to make sure you have enough juice until you land.
But it's expensive and there are plenty of cheaper options. Mophie sells a similar pack that's $10 less, though I haven't tested it. And if you really want extra life without the cost, just get buy a power bank on Amazon.
In the end, the case is great if you're going to spend a day or two away from an outlet and could use the extra peace of mind.