This is the road that Darwin travelled on (there is a plaque by the road in memory of him). Passed Villavicencio the road is unpaved but in good condition. We saw very little traffic (which is a good thing) and what we did see were motorbikes. The road is very twisty and they call it the road of the year as it has 365 curves in it. Great scenery and you will see wildlife, we saw foxes, condors and loads of guanacos. Most traffic seems to go from Villavicenio to Upsalatta, I say this as we didn't meet any traffic coming the other way. Eventually to get to a look out point where the road splits and a section goes to the observatory. On a clear day you will get views of Aconcagua. There is the usual cross there and after that it is all downhill to Upsalatta. About half way down you hit (not great) tarmac again. Definitely worth doing if you have a car.

As you leave Mendoza on RP 52 you will quickly be on a long mostly straight road with NO services, so be sure you have enough fuel and whatever you might need. I gains in elevation steadily to Villavicencio where an old hotel build in 1940 is presently being renovated. A stop there and at the visitor center a few kilometers before will cost you about $10 USD per person. After Villavicencio the pavement ends, but the gravel road is quite passable by almost all vehicles. Many switchbacks lead to some absolutely spectacular views. Well worth the drive