Just curious how much room is in the back? I am gathering not as much as the S-Class sedan, but can an average size adult sit comfortably in the back?

Definitely less room than the sedan, but for a coupe it is very spacious. In fact there is more room than in my CLS63 (which has more than my old CLS55), particularly headroom. Unfortunately the extra head room meant doing away with any potential roof opening/sliding mechanism, so the pano roof is just fixed glass. One of my few quibbles with the design, as I like to tilt the roof when parking in the summers here in AZ as it helps vent some of the hot air when I park outside in the sun.

Access is awkward of course without rear doors, but the front seat automatically moves out of the way when the lever is pulled and its tilted forward.

Interesting that a two door coupe has more room than a 4 door, I am hoping that an averaged sized adult won't have issues with leg room etc. I am not sure I like the design of the rear though, just don't like the lights design.

it all depends on the people in the front and the seat adjustment. two tall people in the front doesn't leave that much space for them in the back.