For most of the short distance driving I do the Prius II seats are ok for me.
For long distance driving I have to say I find the seats extremely uncomfortable, for my shape and way of sitting. I find the seats too 'dished out' for lack of a better way of describing that, with too little lumbar support. That combines with the shoulder support being too close to the spine, and standing out too far, which for me results in the shoulders being pushed forward. In conjunction with the dished out shape, by back is just totally curved, and after long driving my back really hurts.
The other day I had a long trip with two 4.5 hr trip sections, and at the end of the day my back was killing me!
I have decided I need to do something about that, and will first try a variety of external add-ons, and if those work I may have a custom seat upholstery place add inserts beneath the leather.
However, for fairness I should add that my wife loves the Prius II seats. My previous car was a 2003 MINI Cooper S (it had height adjustable seats and adjustable lumbar support), and I loved the seats and ergonomics of the car, the best I ever had in that respect, but my wife hated the seats in the MINI.

I wish Toyota would offer more options for the Prius. I would gladly pay extra for seats that have: adjustable height, adjustable lumbar support, heated seats.

M