Yesterday I sat in the back during a test drive in Carmel Valley (Tesla was in Carmel for Concurs weekend and on the streets Friday they were even offering test drives to non-reservation holders who showed an interest - and apparently had a high hit rate of reservations taken afterwards).
Anyway, while the back seat itself is nicely padded, the seat height is simply too low for adults. If you've seen Jimmy Stewart at the Moroccan restaurant in Hitchcock's The Man Who Knew Too Much, you know what I'm talking about - your knees end up being uncomfortably high. I would not be comfortable for an hour drive in the back seat unless I sat sideways - and I'm not particularly tall (5' 11").
It was ironic that while the driver asked me twice if I had enough headroom, I kept wanting to talk about legroom (kneeroom?). It looked to the driver that the rear outside corner of my head was really close to the headliner, but in moving around normally I never hit the headliner (car had sunroof). I guess since the close point is behind the ears you don't notice how close it is.
But, the knee room was bad. Between the high floor of the car from the batteries and the low roof from aerodynamics, the rear seat passengers literally get squeezed. I wonder if all the complaining about rear seat headroom has made Tesla decide to lower the rear seats a bit? That would increase headroom (which you immediately notice) at the expense of knee room (which you maybe don't notice for a little while). This car will definitely not work as a taxi nor any other situation where back seat comfort and egress is important.
For our use, the back seat should be OK. Our son is tall, but on long rides he'll be the only one the back (or maybe he'll want to drive for a change), so sitting sideways a bit is possible. However, if you're intending to have a couple of big people in the back on a regular basis or for long rides, you really need to check the comfort for yourself - you might want to get Model X instead, even if you don't need the 7 seat capacity.
Tesla, if you're reading, maybe in future models the back seat should have a height adjustment so that occupants can find the best compromise for them. Personally, I'd rather lose a few hundredths on the drag co-efficient and have some more knee/head room.
Edit: Here's the only picture I could find online: