I tried it for my last cruise which was a 4 day cruise. It had its pros...
Here are the features I used
priority embarkation
Priority disembarkation
reserved seating in all available shows
the lunch by Chops during embarkation day (this way underwhelming)
wifi
luggage drop off at main dining room during embarkation day
Here are the features I did not use:
Room service (according to the letter this was free)
Priority Time at activities- these were at 9am, which is not a time that I normally wear a bathing suit to ride some waves. Rock climbing was also at 9am. I did not find that the time was convenient and I do not know how many people showed up
For my next cruise in July I decided to cancel the key because I did not feel that it was worth that much money as we would not need to use priority boarding or disembarkation.