From the home screen, and in all of Amazon's built-in apps, you can tilt the phone right and left to bring out contextual menus from either side: shortcuts, recommended items, settings, folder lists, categories, and other really useful things. A quick dip of the phone brings up a selection of system-wide shortcut buttons and your notifications. You can't customize this screen's button layout, but among the buttons you'll find Mayday, which is your shortcut to getting help with your phone from a live person, 24/7. (More on Mayday a little later.)
The built-in apps are capable, but I missed being able to use Gmail or even OWA for my Gmail and Exchange accounts, respectively. Amazon's email app has annoying quirks and just isn't pretty to look at. On a more consumption-focused device like a tablet I probably wouldn't care (I barely ever use the Mail app on my iPad), but on a phone, having a solid email app is essential. And the Amazon Appstore doesn't seem to have a killer alternative just yet.
The rest of the built-in software is all over the place: The bare-bones Messages app is only so-so. But the Maps app is actually really good, with an easy-to-use interface and solid step-by-step directions for driving as well as transit and walking. It didn't steer me wrong once in the four cities I tried it in: Chicago, Milwaukee, San Francisco, and Oakland.
Thanks to the Dynamic Perspective cameras on the corners of the phone, the map's perspective changes as you rotate the phone or move your head, letting you get a close-up view of some 3D landmarks. I found it cool but not necessarily useful, and thankfully Dynamic Perspective only works when you're looking at a zoomed-in map, not while you're trying to follow directions. The Silk web browser, by contrast, uses Dynamic Perspective to auto-scroll long pages, but it was hard to control with accuracy, and I kept losing my place when I lowered the phone or put it down.
What it does well
When the Silk browser's auto-scroll annoyed me, I didn't bother digging around in settings to find how to turn it off. I've gotten far too lazy for that--one push of the Mayday button and I can ask a support tech, who could actually even do it for me. Every time I called, the Mayday helpers came up within seconds. And while they can't see you, you can see them, and they can see and even control your phone screen. It's a great idea, especially when you're getting used to a new operating system, and so far the execution has been flawless (hopefully not just because the installed base is still relatively small).
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