Yes I´m still on 5.5, as 6.0 didn´t have the desired visibility features in the arrange/project window where I spend most of the time.
I have not tried the new mixer and control room, but it looks a bit out of place to me. As if it wouldn´t fit too well into the overall design of the program. Can´t tell about the functionality though.
it is my impression that the old mixer seemed to fit better into the graphic scheme of the program, but my deciding factor to wait with the upgrade was that nothing I really needed was in 6.0, on the other hand 6.5 is PACKED with stuff I´ve been desperate for for a LONG time (years), so 6.5 update will be a day1 purchase for me.
I´d like to know if it will be free to Nuendo users again, or when the grace period for 6.0 buyers starts in case that 6.5 is a paid update.
Yes, yes and yes! 
I want a more seamless appearance of elements and revamped window handling, possibly magnetic docking of elements, but that can wait until 7.0, for now I´m very happy with what is known for the next update