Pretty good, if you need the boost
The Apple Smart Battery Case for the iPhone XS, XS Max and XR is a bulky way to make sure your phone doesn't need a recharge. Apple has done this before, for the iPhone 6S. That battery case was mocked, but it was actually pretty good, if you found its $100 price and slightly bulbous design acceptable.
I've been living with Apple's new and even more expensive $130 iPhone battery cases on an iPhone XS and XS Max over the past few weeks. And you know what? Weeks later, they do the job. They protect the phones, feel sturdy and add a bit of padding. The two battery cases have the same battery capacity (1,369 mAh/10.1 Wh), and in everyday use they ended up lasting just about a whole normal workday, and then dipping into the actual phone's battery at the very end of the day. That means you could use these and not have to charge your phone overnight, if you were ever in a situation where you needed that option. I was on a long flight (9 hours) recently and was able to arrive and still have basically a whole charged phone but with a depleted battery case. Apple claims a combined 39 hours of talk time, 22 hours of internet use or 27 hours of video playback for the iPhone XS or 37 hours/20 hours/25 hours on an XS Max. It's nothing like the massive battery capacity of a stand-alone charger, but in real-life use it's a bit more than I expected.
As to how long the battery lasts over time ... that remains to be seen. Also, how will the case's rubberized construction hold up after a few months or more?
Both cases add enough bulk that the phones seem downright featherweight once you take them back out, but the case's bottom-heavy weight distribution makes it easy to grip with one hand. I've used these on mission-critical trips where I needed access to my phone for photos, videos, audio recording, calls and other features. And they never annoyed me. The silicone casing can be a huge dust magnet and also collects smudges and oil, but it offers bumperlike support on the edges and the back of the phone.
As for removing the iPhone, it's easy to flex the case back and slide it out, and the soft microfiber doesn't scuff or damage the phone at all. Jury's still out on how flexing the rubber case over time will affect it, though.
The cases charge via Lightning or wireless Qi. You could rest the iPhone still in its case on the wireless charger and charge both the case and the phone, or you could leave only the case on the wireless charger (make sure it's properly lined up and charging, though). It takes a good long while to charge both an iPhone XS Max and this battery case via Qi or Lightning via a 5W charger, so definitely consider at least a 12W adapter with Lightning, or something better.
I haven't even discussed the Smart Battery Case for the iPhone XR because, look: You don't need it. The XR already has excellent battery life. I'd even probably skip the XS Max version. The iPhone XS is the phone with the most middle-ground battery life, and it's also the smallest phone. Adding the battery case completely removes any battery worries for me in situations where I'm power-using, like shooting videos or doing lots of streaming or tethering. An XS with the case fit pretty easily into my pockets too. The XS Max with the case is a monster.
I promised I'd follow up with final thoughts on these smart iPhone battery cases a few weeks ago, so here I am... and I'm still not fed up with them. I really like them as functional longer-battery packs -- though I haven't tried other recent iPhone XS and Max battery cases to compare them with, because very few even exist. Mophie's iPhone XS battery case looks to offer similar function at a hair-lower cost ($120). I don't think you'll regret these utilitarian battery cases if you need the extra juice, and are okay with the high price. But consider that you could also easily get an affordable battery pack and plug your phone in to charge in your bag (the Anker PowerCore 5000 costs just $20).
If I had had this case at CES in Las Vegas, it would have stayed glued to my iPhone. At the next major press event when I need guaranteed battery life, I might use it too. In that sense, this is a smart choice for people who depend on their phone for work use. It's a soft case with a big battery glomped on. Do I wish iPhones had even better battery life so I didn't have to use a case like this? Sure (it's why the iPhone XR appeals to me so much). But if you're in need of a guaranteed all-day-plus survival case, this more than does the job.