I've got an iPhone 12 Pro. For my first test this morning, I was able to get my iPhone from 20% to 83% (a net 63% boost in about 2 hours).
I'm on iOS 14.7, I was on wifi, airplane mode was turned off, low power mode was turned
off the entire time, DND was turned off (but almost no notifications came in during that time anyway), and Bluetooth was turned on. The only thing I used the phone for was to periodically check and screenshot the current battery levels.
I'll do some more tests with other configs, like with low power mode turned on the entire time, 5G data, LTE data, etc. But so far this definitely exceeded my expectations of what I thought I would get out of it. Low power mode might help to squeeze a bit more out of it too (I know for sure the phone was definitely doing stuff in the background since the screenshots I took uploaded to iCloud pretty quickly each time). Assuming I can also get at least a 50% boost while on cellular, I'd feel confident having this standing by in my pocket just in case my battery runs low early while on day trips into the city.
Here are my raw data points in case anyone is interested:
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Edit: Additional data, graph