I previously owned a 2008 prius and while the seats weren't terribly uncomfortable, they weren't great. About 3 months ago I sold the '08 and bought a 2012 Prius V. My back has been killing me ever since . . . until now. I called the Toyota dealer in San Rafael, CA to see if they've heard similar complaints and had any suggestions and they recommended I call one of the a well established auto upholstery place in town called Teeters & Schatch. What a difference. Ken the owner sat in the driver's seat for several minutes, had me tell him about any cars I've driven that I thought were comfortable (he's been doing this for 35+ years and knows what certain seats feel like), and had me test out several foam inserts that he held in place. He ended up replacing the bottom cushion with something firmer, extending it just a touch, but more importantly he changed the basic contour of the seat back from concave to convex, with lots of soft cushioning. Not a cheap fix ($450), but worth every penny and my back is loving every minute of it.
Illustrates well how seats are like mileage; YMMV and YCMV (Your Comfort May Vary), with both being addressable via small tweaks.
I use a lumbar support for the lower back area. There is an inflatable lumber built in but it was not nearly enough.