Do not underestimate the potential complications of this border crossing, at 3200m one of the highest in the world. In good weather, things should proceed smoothly, if slowly, because of the immigration and customs stop on the Argentine side of the border (this is for travelers crossing into Argentina from Chile; if you are leaving Argentina you do not have to stop here).
The crossing is often closed because of inclement weather and you don't want to be stuck up here at night without accommodations and warm clothing. Between June and September, the crossing is closed from 9pm to 9am. Check www.twitter.com/upfronterizos for the latest conditions and crossing times.