So I've had a chance to play with the cameras in both my 6P as well as in a 5X (which are exactly the same aside from some software-driven differences). So far I am very impressed. There is virtually no shutter-lag whatsoever and the focus is incredibly quick. Burst mode works very well but it is easy to accidently do, especially if you are used to your camera-phone taking much longer to focus. The performance is impressively good in low light as well. The only issues I've had so far with the camera is that the flash makes things a little washed out and the stock app is very rudimentary. Also the app defaults to HDR on which is fine for still shots but I recommend turning off for shots of movement and/or people. I would say overall so far the camera has exceeded my expectations, which were both fairly optimistic and also pretty technical since I have a background in photography.
As far as the app, I've been dabbling in a few 3rd party apps to look for something with more user-control and the one I've liked so far is Camera FV-5. Allows you to shoot in RAW, you can map focus and the shutter to the physical buttons which I think work better than tapping the screen (less camera shake) and of course a lot of manual adjustments on settings such as ISO and shutter speed.
I still recommend something like a DSLR or mirrorless camera for more fancy/artistic stuff, but as far as an everyday snapshot taker, I think this is the first time I've felt a camera-phone could replace a dedicated point-and-shoot camera. Oh, and if anyone cares the selfie-cam takes great looking photos but does have a touch of shutter-lag, I'm assuming because it doesn't have as good of a focusing mechanism as the rear-facing camera.
As far as the rest of the phone, I'm pretty impressed there as well. My favorite feature may be the fingerprint scanner which is both quick and accurate. It actually loads the animated gif thread faster than my Alienware computer (which admittedly is 5 years old now). Screen is so bright I keep it at about 1/3 brightness all the time and battery has been lasting from ~14 hours-ish of what is heavy use for me with battery to spare. I haven't had a chance to delve deeper into some of the additional features of Marshmellow yet so no comments on some of those.
My major complaint is the physical button placement. Both the power button and volume rocker are on the same side of the phone about 1/2 way down the phone. With the way I naturally hold the phone it is pretty easy to accidently bump the power button and turn my screen off. Not so much of a problem in portrait orientation, but if I'm holding it in landscape and the buttons are on the bottom I accidently hit it all the time.
