I had The Key for Anthem of the Seas, March 23rd sailing.
There was a priority Key check-in line, it went very quickly. There was no priority boarding. Once on the ship, The Key had a little check-in table, easily located. There, I was given a folder containing the details of Key perks for that voyage and Voom log-on instructions, and directed to the Theater if I wanted to drop off my carryon, and then to go to my choice of Chops or Jamie's Italian for The Key lunch.
I went to Chops, waited almost no time at all on a fast-moving line, and was seated. Wait staff was quick and efficient. and my lunch (I had the filet) was excellent.
Rooms were ready at 1:30, there was no priority treatment there.
There were sections of the theater roped off for Key guest access for all shows I attended. Port debark was quick at the ports.
Final debark was blindingly fast - escorted past the very long line (I almost felt guilty), whisked to the Key luggage pickup section, and was through and out waiting for my ride in a total of seven minutes from the time I had finished breakfast.
I did not take advantage of any of the special times for onboard activites.
Was the program worth it to me? Yes, I think so. I don't feel ripped off, in any case.
Will I get it on my next cruise? No. Now that I've had that experience, I don't need to have it again.