I called the Hertz rental counter at the San Jose airport last night to ask if they had any Priuses for rent; they said no, but call back in a couple hours. I did; they said no, but call back in the morning. I did; they said yes!
So I went in and rented one. (After some wacky interactions with the wacky rental guy, who decided to tell me his opinions of the looks of various car makes and models; for example, he told me he was glad the Ford Escort was no longer being made, because it was ugly.) The Prius is a 2005 model (I think; forgot to check), black, without any of the option packages.
And somehow the seat is a whole lot more comfortable than the one I test-drove or the one I sat in a couple of weeks ago without driving.
The headrest is still mildly uncomfortable, but not too bad, and I can actually make it pretty comfortable by turning it around front-to-back. Though I'm not sure how safe that is.
The headroom in the back seat is indeed as low as I thought. If I sit in the back seat and lean back, my head hits the ceiling. And I'm not all that tall, not compared to the tall people I know.
But the headroom in the front seats is fine.
I'm not sure yet about the visibility issue. The split rear window does bother me, as does its small size, as does the amount of general visibility that's cut off by door posts and the like. The blind spots on this car are much bigger than I'm used to. Still, I'm a very cautious driver; I'll just have different things to be cautious about than I'm used to.
The keyless entry system (which this particular car doesn't have) looks less and less practical the more I read about it. But it's a fairly expensive extra, so that's probably just as well.
Backing up is a bit of a problem: low visibility, and in daylight there may be reflections on the rear window that reduce visibility further, and I hear that at night it's even worse.
And the "dashboard" area still looks weird to me, as if it's being projected through a distorting lens, like maybe one of those fresnel lenses from Brazil.
Still, all in all I like it. I think I'm leaning pretty heavily toward getting one. I'll keep driving the rental for a few days, and I'll see if I can do another test drive of a Civic (and see if the loud whining noise was unusually loud on the one I test-drove before, and if the seat of another one would be more comfortable), but this is pretty tempting.
At first I didn't like the overall look of the car; it looked kinda buglike. But now I think it looks kinda cool and futuristic.
Anyway, we'll see.