I visited Iguazu Falls in 2003 and, based upon what I had read and what I had been told, I had high expectations for the falls.
That said, after getting off of the bus, while I was walking to the falls, I heard a tremendous roar of the water. I got chills, the roar of the water was so loud and pervasive. I ended up spending two days on the Argentinian side of the falls and one short day on the Brazilian side of the falls.
From Brazil, one gets an overview of the falls and the river (flood plain?) leading to the falls.
From Argentina, you are able to hike down several trails and explore the area much more closely. You can also take a boat ride which takes you close to the falls, where you will get wet from the spray.
I took the boat ride and, while I don’t do a lot of the average touristy things many others do, this was well worth the experience. If you stay at the park until closing and you walk some of the more remote trails (which I did), you will get to see some of the area’s animals which come out once most people are gone.
I will be visiting the falls again this year (2010); that’s how spectacular they are!