Well the free year of Prime does help the value proposition - if you're not an Amazon user, this phone isn't for you, but otherwise $100 subsidized is a bit more palatable. However in comparison to the Moto X and OnePlus One, it feels like this should have been $100 before the year of prime, with Amazon trusting greater engagement with their ecosystem to drive profits.
Firefly is interesting - less about sensors and more about smart data detectors and uses of big data - so they absolutely should release an iOS and Android app, especially as it always drives you to a product on Amazon.
Dynamic Perspective feels a little gimmicky and 4 cameras always looking at you when the phone is on, even in the dark with infrared sensors? I know there's nothing malicious but that made me cringe just a tad.
The motions + head detection may create reliable motion control, but the gains seems to be pretty trivial.
Overall - it's good to see some new thinking, but not exactly a must-have. I'd expect this to be free on contract in just a couple months with some firesales closer to the holidays.