I don't have it yet, but from what I've read, the quick camera launch sounds like an awesome, useful feature.
While I typically prefer on-screen buttons and no physical home screen button, it's pretty hard to argue against double tap to launch the camera from anywhere. That's a really good addition.
Plus I like the fact that it launches in 0.7s as well. When you need to take a photo quickly, fumbling around looking for the app and then waiting for it to load is not fun. Seems like the overall camera experience with the S6 should be really good.