I am a big supporter of iA Writer on the iPad, and bought the Mac product. That was a (big) mistake at the price.
On the Mac, with it's very different form factor, iA Writer simply does not shine.
The Mac form factor, especially with a large screen, requires more flexibility. For example, I would like to sometimes view the text in a larger size (my eyes are getting old). I can't. Further, with a large screen, I would like to adjust the margin sizes, top, bottom and sides. I can't.
But, on a MacBook Air iA Writer will no doubt shine pretty well. The smaller the screen the better. But I use 24-27" monitors, quite different animals.
Frankly, Pages in full screen mode is pretty terrific. It provides all the flexibility (simple or sophisticated) I need, albeit without one or two nice features found in iA Writer. But the flexibility afforded by Pages makes it the winner on the Mac, not iA Writer. iA Writer seems a big waste. I use Pages, TextWrangler, and (rarely these days) Mellel. Word I ditched a long time ago - though I still own the latest MS-Office (also prefer Numbers to Excel, and Keynote to PowerPoint).
I will stick with iA Writer on the iPad, and right-off the Mac version purchase as a bum investment. Sorry guys.