We visited these vast incredible waterfalls in February 2010, while on a trip to other parts of Brazil and Argentina. Apart from the hot/humid weather, this was quite a heavenly experience and a highly recommended visit for everyone.
Just a note to make sure you mind your visa situation before you go, as it’s a bit complicated here. The falls are shared between Brazil & Argentina, with airports on both sides. Make sure you arrive/depart with the right airport, and if you have a single-entry visa to one of the countries and cross the land border then you can’t go back!
Pictures below from a helicopter tour over the famous “devil’s throat gorge”, then a ground walkaround of the falls