We just got home tonight from the Fantasy and stayed concierge in cabin 12522. My answer of whether or not it is worth it is, it depends!
From a financial perspective is it worth it? No, not strictly speaking.
From an experience perspective, it is totally worth it.
So if you are coming from a place where one needs to get the most bang for the buck, concierge isn't going to make you happy. But if you want a smooth, high-benefit experience, it's very worth it.
These are the things that I enjoyed the most:
-Not having to stand in line and deal with Guest Services for anything
-not having to go grab our photos we ordered on the morning we were disembarking (concierge had them)
-a quiet lounge to drink the dozens of lattes and teas we enjoyed
-a quiet hot tub with champagne, wine and beer nearby
-not having to pack sunscreen, slippers, or a robe.
-the ability to book Palo and Remy just weeks before we sailed (we booked late), as well as character breakfasts
-being able to have someone change our dining reservations and nursery reservations, and just have it done
-not having to go to Future Cruise sales or whatever it's called to book future cruises (concierge handled that)
-endless bottles of still water for me and sparkling water for my husband
-they tracked down a lost toy for us, saving us hassle
-the ability to be walked off the ship at disembarkation - we didn't do this and I regret it now
-our luggage was in a much smaller area and so was easier to find when we got off the ship.
-snacks all day and an ability to grab a quick breakfast and avoid busier places
-better pillows
-the sundeck was relaxing and a really nice place to hang out. If I wanted an ocean view, there are doors leading out to the rest of deck 12 and it's easy to come and go
-the concierge staff was very nice to the kids in the lounge, allowing us to hang out more casually and chat with other guests, as the concierge hosts chatted up the kids
Concierge felt like a hub for us. The ship can be very crowded and busy, and the concierge lounge was quiet, serene, and comfortable. We met some very nice fellow cruisers and it was nice to see them again and again as we passed by.
Everything was just so smooth with concierge assisting us. I lost a room card, for example. It was replaced in about 90 seconds, no hassle. The kids club forgot to send our kids' art work - it was waiting for us an hour later in our stateroom. My husband forgot nice pants - a tailor visited us 20 minutes later and had a pair of pants for my husband to borrow. That kind of stuff. It was lovely.