The 787 seats are tight with someone big in the middle. I just did it on DEN-NRT and I was leaning into the window or into my seatmate. Given that I wanted to be asleep and that I like leaning against the window to do that it wasn't horrid but it also wasn't spectacular. I don't actually think it would have been any different on the 777; that's just the nature of a big guy in the middle seat IMO.

As for the in-flight experience, the 787 was noticeably to me. The increased humidity, especially, had a notable impact on my enjoyment of the flight and, more importantly, my time on the ground after landing. This was my first longhaul 787 and followed on a comparable amount of flying the week prior and the difference was real for me.

My trip report from the inaugural DEN-NRT (and return the following day), including some photos and more details, is online now.