I like the priority embark/debark. It has worked well for us. The coupon book doesn't matter. It was never that great and now is pretty useless. I also like the diamond lounge and the new diamond breakfast perk. Granted it was the Oasis on it's first revenue cruise, but last December we D and D+ members were showered with attention. I almost needed another carry-on bag to carry all the gifts we got. They definitely made a good impression on me.
Obviously it is better to be diamond on the Freedom and Oasis class ships. It is a diminished thing on the older ships. But don't forget the C&A perks go beyond the ship. Free air deviation if I use RCI air is a good perk, although that one starts at Platinum level.
However...since we book suites on Carnival we also get priority embark/debark even though we haven't hit the magic number 10 cruise yet.
I am quite interested in trying one of the old R ships that Princess acquired. Being small, the value of priority embarkation/debarkation and other things would tend to be less meaningful to me. After all, when you have 700 passengers to move it becomes less of a problem.