Of all the new phones for which Apple built these battery cases, the iPhone XS stands to benefit the most: When we reviewed the XS, we found that with frequent use, the built-in battery generally ran between nine and 9.5 hours before going completely dry. We found that the external battery adds an additional nine to 10 hours of power though.
Of course, that number is largely dependent on what you're doing with your phone. In my case, I measured the battery's performance while going through my usual routine. That means lots of podcasts and mobile hotspot time while in transit, near-constant Slack and Gmail use, and a spot of gaming now that Langrisser is out, et cetera.
Your mileage may vary, but with what feels like normal, fairly consistent use, the case essentially doubled the XS' battery life. If you're the kind of person who uses your iPhone for a bit and then puts it away for long periods of time, you can expect even better performance. (The same is also true for last year's iPhone X, which works with the XS battery case despite not fitting perfectly.)
Going into this review, I was concerned that the extra weight of the battery would make the already-small XS uncomfortable to use. It's actually not that bad: The added heft never goes unnoticed, but the total package isn't overly cumbersome. That said, the hump plus the grippy silicone mean pulling the encased phone out of my pocket is often a struggle.
I wish I could say the same about the XS Max's battery case. Like its little brother, this external battery adds about nine to 10 hours of use, but the Max just doesn't need it as much. By dint of being one of the biggest iPhones Apple has ever made, the XS Max already has a giant battery that routinely lasted for 12 hours of frequent use.
There's no question the Smart Battery Case does its job well, but it's arguably overkill for most people who splurged on Apple's big phone. (For those keeping track, Apple never released a larger version of its Smart Battery Case for its Plus-sized iPhones, presumably for this very reason.) The idea of being able to use an XS Max for two days off a single charge (three, if you're lucky) is definitely appealing, but since most people charge their phones fully at least once per day, I have to wonder who really benefits from this thing.
And let's not forget that this case makes the XS Max feel pretty huge. You can forget about slipping it into your pocket unobtrusively. I know my share of people who already find the XS Max a little unwieldy, and for them, the Smart Battery case is a nonstarter. It nearly doubles the phone's diameter. Anyone with small hands will find it close to impossible to use one-handed.