What I like about watching back there is that you are surrounded by the fireworks. Some are from the castle, and some are from well behind the park, so you are just immersed in the middle of it all!
Another alternative to avoid the crowds but still enjoy the show is the Poly beach. We did that on Monday, and I am quite sure that they recently replaced or otherwise improved the sound system that plays the music. Other times we felt we could barely hear it, but it was loud and clear this week. So magical to sit in a chair on the beach and watch and listen to the show across the water!