It has been a long time since I used a battery case on my iPhone. (I am pretty sure my last battery case fig my iPhone 3GS.) The reason I gave up on them is that when you get a new phone, the battery case is of no further use to you.
Nevertheless, I ordered one of these. I've got a lot of travel over the next six months, and it sure would be nice to carry an extra charge when I am out of town. With the iPhone XS Max model, this new battery gives you talk time up to 37 hours, Internet use up to 20 hours and video playback of up to 25 hours. The other way I justified it is that when I ultimately hand this phone down to someone in my family, they will inevitably be less interested in charging their phone than I am. This battery will serve them well.
A nice feature of this battery is that it is Qi-certified. That way I can set it on my Qi charger and charge the battery while it is in the case. Ultimately, I expect this will be the battery I use for trips to Disneyland and trips out of town, but not my everyday case. If you are wondering, after way too much consideration, I bought the black one. I think a MacSparky sticker will look nice on the back.