Did the Rio de Janiero to Santiago on the Mercury in March of this year. It was nothing short of fantastic.
What to do?
In Ushuaisa (still can't spell it), the Tiera del Fuego park was spectacular, but leave time to just wander the town and hope the cable cars to the glacier (take the park tour in the morning if possible, leave the afternoon for on your own).
Buenos Aries, a lovely city, some excellent shopping due to their depressed economy. Make sure you see Eva's grave, the palace.
Puerto Madryn - we took the excursion to the Penguin Rookery, a long drive but well worth it.
It is a most interesting trip.