We have had the Key many times. The main dining room IS open on embarkation day for Key holders....that is where the (Chops) Key lunch is held. The menu on disembarkation morning is also a special one for Key holders.
We love having the Key and it's always been worth it to us: internet, priority seating area in the terminal and priority embarkation, Key lunch, carry on luggage taken to our cabin for us, reserved seating in shows, priority tender access going ashore (a huge benefit on our Greenland cruise when the lines were so long for those wanting to go ashore), priority disembarkation.