Being able to quickly launch the camera on your phone is the best. Watching people unlock their phone and tap the camera icon on their home screen while you sit there already recording or snapping away because a gesture or button combo let you launch the camera as soon as the phone left your pocket is amazing. Any time there's a way to quick launch the camera on a phone, I immediately learn it and make it a part of my workflow, because it really does make a difference when trying to get that quick picture.
Google was not the first to implement a quick-launch command for the camera, but the way they have the power button set up to work as the camera launcher is something that could be universally applied to all Android phones in Marshmallow. Unfortunately that isn't happening, and it sucks.
Using this double-press on the power button to launch the camera seems like a win for everyone.
Between Samsung's double press of the home button, LG's double press of the volume down button, and Motorola's screwdriver-style twist to launch the camera, Android users have a lot of options out there. And there's a argument to be made for keeping those options available in Marshmallow so users who have been using that feature aren't displaced just because Google decided they had their own way to do things.
For some reason, none of the manufacturers who have already rolled out Marshmallow to their existing phones have decided to also add the power button double-press. The existing launch mechanisms are there, and they still work well, but Google's method didn't make it into these versions of Marshmallow. And the double-press of the power button isn't being used for other features on these phones.
Everyone knows that manufacturers can take Android and mold it to fit what they think is best for their users, but double-pressing the power button seems like a win for everyone. Its a quick physical shortcut Google could have advertised as something "uniquely Android" and had some fun with. While it's easy to see this as a "free" feature, having multiple shortcuts could have caused other problems when educating consumers, so that decision makes a little sense. If you look at other manufacturers that have no quick camera launch, like HTC with their Marshmallow-based A9, this decision makes a lot less sense.
It's unlikely we'll see many manufacturers follow Google in this, for reasons that aren't clear at all. It works remarkably well on the Nexus 6P and Pixel C, and while the Nexus 5X stutters on this command occasionally that has a lot more to do with the unoptimized state of that phone in general and not specifically that function.
For manufacturers like BlackBerry, where there's currently no mechanism to launch the camera from anywhere, it seems like an obvious win for everyone. Who knows, maybe these companies are waiting to include the feature in their next wave of phones that launch with Marshmallow. Stranger things have happened.