I have a three seat middle row and I can tell you the rear view is terrible. I can't believe they didn't make the middle row seat head rest fold-able or removable. The view is about 25% obscured - and it's the most important 25% of your view - the middle - just awful!
I love my model S and am working through getting accustomed to the differences with the Model X - but I gotta say, this oversight is awful.
I'd liken it to having a best friend whom you see several times a day - and he's got an un-removable booger hanging from his nose. Every time you look at him, you have an urge to get him to 'fix' it but there's nothing you can do about it. Except look at it... several times a day... and wish you could change things....
The scary thing is what if you're about to get rear-ended by a car behind you but you can't see it... simply because you're looking at your middle seat head rest. Major safety issue in my opinion all joking aside.
I have included two photos so you can see for yourself- one with all headrests up and the other with the 3rd row folded down.
My workaround is to keep the camera running on the bottom of my screen but that gobbles up half my screen - just to compensate for the mirror.
Hope this helps - and I hope they will do something about this. I would gladly ship my car back if they wanted to swap out my middle row for a new seat with removable headrest.