The battery life on the iPhone 6s Plus is amazing, you can easily use it for a whole day and still have 30-40% battery life left when you go to sleep.
But it still is not as good as it is on my iPad Air 2. You notice that if you do some hard work like I do every day. Blogging, video editing and audio editing for example. Then it is a bit worse than the iPad, and that's too bad because I wanted it to be as good at least. I use my iPhone 6s Plus as my main computer now instead of the iPad Air 2, so that's very important to me.
I can do between 6-9 hours of hard work on the iPhone 6s Plus, so maybe I shouldn't complain. It's still better than most laptops. But my iPad Air 2 is even better than that.
But most people will be more than satisfied with the iPhone 6s Plus battery life, it's incredible in normal use. Not all are using it as their main computer and for such demanding tasks as I do.
Below you can see screenshots of my battery life at different occasions: