I have hiked Grand Falls on numerous occasions and every time it is different. Be prepared to get dirty and the Moenkopi does not wash out of your clothes!
I have never been disappointed on any visit to the falls, whether it be wet or dry. When the Little Colorado is not running it is really awesome to hike around and see what the water has done to the limestone deposits. One interesting feature is the cylindrical holes that are created by the water. During a wet visit, a friend was walking across the first tier and I warned him about these holes. Next thing I knew, he fell down into one and came up covered in what looked like chocolate milk (my poor upholstery!). When the falls are at a trickle, you get the chance to stand under the water or even venture behind the falls and get a view of some beautiful rainbows. During the monsoon season, I will call up my friends that live in Leupp and ask them is the Lil Colorado is running hard. During peak flow, you can stand on the rim of the canyon and get completely covered in silt.
I have also been to Grand Falls during some of my geology classes from NAU. Interesting geology!
One of my favorite places!!