We had no idea how FP+ worked with Fantasmic. We thought it would work great to get one, since we did RnR and ToT twice already. So we got them for Fantasmic, then went to dinner on Boardwalk. We were in no hurry to get back, figured no problem we have FP+.
Well....we arrived at Fantasmic about half hour beforehand, and there was a huge line that wound way down past ToT. And that was the FP+ line, not general seating. The line was so long dozens upon dozens of people just jumped in the line somewhere, instead of going to the end. We literally felt like we were the last ones in line for what seemed like forever. We finally made it in, but it was packed. Found seats at the bottom though, 2nd to last section on the left. They were fine, but lesson learned. FP+ does not guarantee good seating. You still have to get there early, which sort of defeats the purpose of FP+ IMHO.