From the perspective of an XS updated from 12.4.1 to 14.0:
-It's genuinely surprising how snappy 14 is. I think it may even be faster than 12 in most UI actions. With past OS updates, you'd only hope to maintain the status quo of the version the device shipped with, and not have it get any slower, but to have the opposite happen is quite welcome.
-Widgets are a neat idea, but too rigid for my taste. I could find the Weather widget to be useful, but it jumps from compact square to rectangle, to massive. The square is nice for a quick glance, but that kind of info is easy to obtain. The medium rectangle would be fine, but it gives an hourly forecast instead of a daily forecast. To get a more useful five-day forecast requires the massive, space hogging widget, which crams everything in.
Why aren't there more choices? I certainly hope that it's not limited to three maximum. And why can't they appear on the lock screen, like on Android? Who cares if it's copying; nothing wrong with copying the good ideas.
-Similarly, the App Library not being configurable, even just to change the default to the list view over the category view limits its appeal to me. The categories offer nothing that folders didn't before, and worse, they're rigidly dictated by Apple's categories, and developers' sometime puzzling classification choices.
-Speaking of stealing good ideas from Android, the Back Tap actions are a great example. But it's buried in the Accessibility settings and not easily discoverable, and not made any easier by the gazillions of settings options that keeps growing. Going through them after each update is a chore, but given the random bit flipping that can occur, it's worth the effort.
-One of the reasons I avoided iOS 13 was the problematic native Mail app. Even one of the later 12.x updates slowed down Mail, and gave it all sorts of weird pauses, but it seems that 14 has made Mail responsive again, at least for me.
Battery life is a question mark, but 2.5hrs of active screen time since installation has consumed 20%. Standby consumption TBD.
So far, the only disappointment is that 14 seems to consume about 2-3GB more space than 12.4 did. Such growth is not unexpected with added features and such, but can be precious on a 64GB or smaller device.