The back doesn't lie flat. I added some extra room at the bottom for my big butt and hips, and while the top yoke has a flat bottom, there is a slight "curve" to the bottom 3 panels of the back, giving the back some shape.
I'm really enjoying sewing up this jacket. Denim is so easy to work with. It doesn't ravel, so after the initial fray, you don't have to worry about the edge finish. However, I did use my serger to finish the edges.
The top stitching is a bit of mindless work, but it looks really nice.