Not as an "all in one" but, for me, it does replace the need to have an iPad or a tablet at all.
I'm a writer, so I can't get rid of my computer altogether - that'll never happen anyway, as my house is filled with varying kinds. When my last laptop that I'd been using, finally was too cumbersome to carry - and frankly just getting a bit slower than I could deal with - I considered the Surface and iPhone 6 combo, but I had some gripes with the Surface. Anyway, to make a long story short, I went MacBook Pro. Since I don't like the idea of having to switch from laptop to desktop dealing with my manuscripts, I figured go for the more powerful device.
But then there are times when I'm just hanging and I hear dialogue or some other things come to me and I need to hurriedly jot it down. I went for the 6s Plus last week for that very reason. I write a slew of notes and I really didn't want to have to bring all of this stuff out on a daily (we have several children, including an infant, and this would defeat the purpose of why I even went for a smaller laptop to begin with). So for me, the Plus/MacBook Pro combo works well.
I always say keep it simple