Here's an additional review I found that gave details that I didn't know about:
from the article:
According to iFixit, the Smart Battery Case holds a capacity of 1877 mAh. If we compare that to 1715 mAh in the iPhone 6s and 1810 mAh in last year’s iPhone 6, a logical estimate is that the battery case should provide roughly twice the battery life of the standalone phones.
In my testing, I’ve found this to be true, with some caveats. The Smart Battery Case isn’t optimized to charge the iPhone, but to power it. Apple clearly wants people to put the phone in the case and forget about it, charging both batteries simultaneously through the case’s built-in Lightning port.
When used that way, you can definitely get more than twice the battery life of the standalone iPhone. That’s because the case will power the phone and drain its own battery first. It’s only when the case’s battery is completely depleted that the iPhone’s internal battery starts to discharge.
This behavior presents a very specific but significant advantage, one that has much to do with the fact that charging a battery is an inherently inefficient chemical process, and a non-trivial amount of power is always lost to this inefficiency when charging a battery.
You can minimize these power losses by starting off with your phone’s battery at 100%, because the case’s battery will not be used to charge the phone then, only to power it. There’s no charge being transferred from one battery to the other one in this instance, which is where the biggest losses occur.
I managed to get over three full days of regular usage out of my iPhone 6s with the Smart Battery Case after unplugging it from the outlet with both batteries at 100% charge, and leaving the phone in the case at all times. That’s certainly more than twice as long as my iPhone lasts by itself.
By comparison, if I drain the phone’s battery first and only then put it into the case, I can get the iPhone back up to approximately an 80% charge before the case’s battery is fully depleted. This is a perfect example of the differences between charging a battery and using it to power a device instead.
If you want to get the most battery life out your iPhone, put it in the case when it’s fully charged and leave it there. You won’t be disappointed.
If you only want an occasional top-up charge, the Smart Battery Case can definitely help, but you won’t get as much juice out of it that way. If that’s your preferred usage, there are other cases with much bigger batteries inside that I’d recommend instead.
This is another reason why I prefer to pay just a little more for the better integration with the iPhone, despite that there are higher powered battery packs for the same cost.
Yes, ideally battery technology would be far advanced enough that we wouldn't need a battery pack for the thin-ness that is desired (or at least some of us desire) for a phone. But until that advancement, this is for me an acceptable stop-gap measure.
Now, to figure out if I should order an XL and a Regular size stand, or just one or the other. @waytools and @support Is using a metal nail file a good tool to get the XL stand size to fit the Smart Battery Case?