Writing on iOS, at least for me, seems like it is too difficult given the smaller screen and keyboard. Granted I have an iPad Mini and could use the BT keyboard, but then, why wouldn't I use the MBP instead? But I can still see this as being useful for others.
When I wrote my book recently, I started off using StoryMill by Mariner Software, but then finished it off in Microsoft Word. I recall looking at Scrivener at one point a while ago and perhaps I should give it another look (the Mac version) for the second book I'm working on.