There were some plot holes (things I wondered about.) Why didn't Horton escape from the circus himself? Can the Whos hear everything in the big world?
I thought that Gertrude was a funny addition to the story. I also loved how she stood up for Horton, but I didn't like how all she thought about was Horton. She very greatly depended on him and there was very little character development for her.
Overall, Seussical the Musical was pretty good.