April is when the Falls are in Peak flow. There is a tremendous amount of water going over the edge, but they are definately worth seeing. It is one of the best times to see them because of the sheer power of the water.
Different view sites along the Falls mean that not all the view points are obliterated with spray.
This is a good time of year to view the falls from a Helicopter, they are spectacular from the air at this time.
The rainforest is fantastice aswell
So each time of year has its merits and demerits, but I would definatley still recommend that the Falls are worth seeing in April.