Have been twice and both times the theater was full. Neither time were we at the front of the FP+ line, first time because we thought we had the advantage over standby and once because I couldn't get us across the park fast enough (DS is disabled and moves slower). Each time it was frustrating because while they opened the FP+ doors first, they proceeded to open the Standby doors not 5 minutes later. I was far from getting in and honestly wasn't sure we would get a seat. The Standby line flows freely and quickly into the theater while the FP+ line moves slowly as they scan our MBs. The second time was worse than the first. A CM saw my DS, stopped me and immediately placed us in the handicapped section. I suppose it can change from show to show depending on the CMs but if I were to do it again I would be waiting at the FP+ line at the earliest entry time.
The show is cute, the kids love to sing along, the historians are a hoot with adult humor and Anna & Elsa are in it for a few minutes. I was more surprised that they were minimally in the show more than anything. I think for their fans it is a good stop but it's not a must do for us.