The Iguaçu Falls are an awesome sight as tonnes of water throw themselves over cliffs and the mist rises amongst the jungle. They are taller than Niagara Falls, and twice as wide, for which Eleanor Roosevelt is said to have exclaimed on her first sight of the Falls: "Poor Niagara!"
It is well worth spending a day on each side of the falls, especially if you plan to do any of the boat rides or other activities offered.
Don't just rush past the main viewpoints and leave. It's important to get a good perspective on the park overall to appreciate this awesome sight.
Whilst the majority of the falls are in Argentina, a better overview is had from the Brazilian side.
boats
Go in boats on the river
brazilian side
On the Brazilian side 37 R$ pp, cheaper for Brazilian residents you get an excellent overview of Devil's Throat and the rest of the falls, from both above and below.