Yesterday I went to test drive a MB E350, since I've read that they are comparable, and share some components. Some of the things I liked are the seats and the drive direction selector on the steering column. I did not like the downshifting of gears during accelaration (which is of course a problem with any non-EV). But the absolute deal-breaker for me was that the top of the windshield, sun visor and rear view mirror were right in my face, no more than 3 inches away. This made me feel very unsafe and cramped, and requires turning turning my head to see anything in the mirror. In the Volt and Prius there is a good 12 to 20 inches from my face to the visor and mirror. Did anyone who sat in the Model S pay attention to this aspect of the interior? It's hard to judge from the design studio's interior side view pictures since the camera position is so close to the car. But it looks to me as there may be a potential problem.
I test drove the same car last year and don't remember the sun visor and rearview mirror being 3 inches from my face but this isn't a problem with the Model S.
I find it hard to believe the E-Class will have less headroom than the Volt and Prius. Was the seat all the way forward? The front headroom for the 2012 E350 sedan is 37.9 inches. The Volt's is 37.8 and Prius is 38.6. Model S is 39.8. Keep in mind headroom measurements are from the seat bottom measured to the roof. I think you might have to sit in the car in person to tell, because there isn't really a reliable way to tell otherwise (in terms of visor/mirror position).
You have to remember that headroom, as measured, does not have anything to do with the space to the windshield and sun visors, but rather measures room from the seat cushon to the roof above the seat cushon. One of the best examples is to sit in a Honda Pilot. Although headroom is similar to other SUVs, the Pilot windshield ends higher providing much greater visability. I am always amazed how much better I can see in my wife's Pilot - I never have to duck down to see trafic lights like I do in other cars/suvs, and I'm 6'3". Having said this, I did not feel closed in when we sat in the S Beta. I may not get the sunroof (I never really open one anyway), and that usually adds an extra inch or two of headroom.