I can definitely jump in here.
I've been using fixed back seats in my daily driver from 2002 till now. They were first in my Audi Rally car (which I drove daily) and have since been moved to the bug for daily use. I personally LOVE a well fitting, properly adjusted fixed back seat...and I don't ever find I need to adjust them. I'm not just using them for short trips either, I do 6hour days without problems, and regularly do 1000km weekends. It does take some time to set them up "just right" in terms of tilt for over-all comfort and enjoyment.
I do still have my stock '69 seats, which I swap back in on big travel weekends where we are stuffing the car full of gear. Getting mountain bike gear bags into the back seat with fixed backs is too annoying. But truthfully, the stock seats are just collecting dust.
('scuze the fresh coffee stain)
-Dave