It is a fun place to eat. Milkshakes are great and as soon as you are seated they bring you fries and onion rings, then take your order. Nickel jukeboxes are filled with great oldies and at times the waiters sing and dance to the music. Just a nice change of pace from the usual dining venues you find onboard a cruise ship.
My wife and I felt an obligation to go to Johnny Rockets while onboard the Explorer and quite frankly we were not impressed. Fast food is fast food and I saw nothing really special. I did have a rootbeer float as I dearly loved them growing up so many years ago but it wasn't anything I couldn't do at home. I was really hoping that they would have a "real" soda fountain but alas, all they had were the floats and milkshakes.
We ate at Johnny Rockets during our Voyager cruise in January. It's certainly a different "cruise ship" dining experience and fun. The food is enhanced by the 50's atmosphere. I love vanilla and chocolate cokes and it's not easy to find a place that has a real soda fountain that prepares them. The shakes and malts are good as is the rest of the food. It's a good change of pace while sailing. Be sure to try it for yourself.