Authorea is perfect for me. I love how I can chunk my writing into independent sections; they are easy to maneuver through, using the index on the left. I love that I can just drag and drop sections when I want to change the order. Getting started is easy - instead of making an outline and watching it expand into a big mess, I just create a block for each section of my text. I don't know why, but that makes it so much easier for me to tackle one section at a time. Adding images and tables is a breeze, and the fact that the captions travel automatically with the images is great because I like to write with them inline, but for some journals, they have to be submitted all together at the end. None of these are actually issues related to "collaborative" writing - Authorea just helps me write. And, when I convince collaborators to give it a try with me, they also find that it makes writing easier for them.