To get to Buenos Aires from Iguacu Falls, you must catch a bus from Puerto Iguazu in Argentina. If you are staying in Foz do Iguacu in Brasil, just catch the bus that crosses the border into Argentina. It will take you to the bus station. There are many direct buses each day from Puerto Iguazu, but the last one leaves around 5:30 p.m. If you miss this one, as I did, your only option is to take a bus to another city such as Corrientes. This should not be an option, because it will take at least 24 hours to get to Buenos Aires. The bus will stop in every town, no matter how small. And the drive through the Pampas gets awfully boring.
To get to Argentina's side of the waterfalls, wait along the main street in Foz do Iguassu for the bus. It cost $1 and will take you to the bus station of Puerto Iguazu. From there, there are many buses to the Iguazu National Park. Buses run on a regular basis. Yes, taking a taxi would save some time. But it would also cost a lot more money.
On arrival at the airport I picked up a taxi to go to my hotel. The taxi driver offered his services for the next day 10 am till 6 pm (probably could have got longer without any haggling) for 150 Reis (about £30). He was a very nice helpful guy with enough English to get by. The following day I hired his car again, this time with his Dad driving.
The service was excellent since they act in a semi-official capacity as tour guides bypassing queues etc.
This works almost as cheap as taking an organised tour (if there are 2 people) and is much more flexible. For example to get to the Argentinian park from my hotel, in Brazil, on a tour costs 60 Reis per person.
I don't want to publish his phone number on the net but if anyone would like it drop me a line.