We enjoyed our experience at Johnny Rockets, but they are certainly going through growing pains. When we first went to the restaurant, it was pretty empty but we were told there was a 45 minute wait because they kitchen couldn't handle a large amount of orders and was backed up. They do have a system where they send you a text when your table is ready, so at least you will know when to go back to the restaurant.
When we came back, there seemed to be some confusion about where they were actually going to seat us as some tables were apparently not "ready" (even though they were all empty and clean). Our server was friendly, but there were some long wait times for simple things. All of our food was correct (and tasty) and we all got a laugh out of the "ketchup smiles" they made.
One person in our group had the chicken salad, one a chili dog and two of us burgers. Burgers were tasty and there is a nice variety but the burgers themselves are kind of small for the price that you pay.
They have only been open since the end of June, so I truly believe a lot of it is just trying to get the right system and work out kinks. I hope that it will improve with time and training.
If you are looking for a place to cool off and relax in the middle of your day at Kennywood, Johnny Rockets is a good option. Just know that it will likely take a considerable amount of time.