In the course of creating and updating my guidebooks to Argentina, Chile, Patagonia, and Buenos Aires (including coastal Uruguay), I have normally traveled by automobile. The distances to be covered are great - Argentina is the world's eighth largest country, only slightly smaller than India, and Chile's territory extends from the tropics to the sub-Antarctic. The destinations are numerous, and many are remote, with little or no public transport. Given the compressed time frame in which I have cover this territory, the car has proved to be my most efficient means of transportation; once in country, I rarely fly, though I occasionally take buses and ferries (as I'm writing this, I'm on the Navimag route from Puerto Montt to Puerto Natales, with my car on board, and I usually travel from Buenos Aires to Uruguay by ferry).
In nearly two decades of guidebook writing, I've thus driven hundreds of thousands of kilometers - an update of Argentina, for instance, means a roughly 15,000-kilometers (9,000-mile) road trip. I've never aspired to the kind of experience that Tim Cahill  chronicled in Road Fever  - driving from Tierra del Fuego to Alaska's Prudhoe Bay in 23 days to make the Guinness Book of Records - but I do have to cover lots of ground in a short time.
From the first, I realized that renting a car was impractical - or rather, simply too expensive for a job that combines long hours and low pay. Since then I've gone through five cars: 1) a 1969 Argentine Peugeot panel truck that belonged to my father-in-law; 2) a 1979 Toyota pickup that I shipped from California but donated to a Chilean non-profit partly because the cost of shipping it back would have exceeded the vehicle's value; 3) a 1988 Toyota 4WD Xtra Cab that I did ship back to California; 4) a 1992 Nissan Terrano/Pathfinder that I purchased in Santiago but wrecked (with extenuating circumstances) on the Carretera Austral ; and 5) a 1996 Nissan Terrano/Pathfinder (pictured above) that I purchased to replace the former and have now had for five years.
While short term visitors to southern South America find it most convenient to rent a car, those who are spending at least four months should consider purchasing one - presuming the car is in good condition, this can prove as cheap or cheaper than renting. Buying a car, though, requires caution, and not just because it might have mechanical problems. There can be bureaucratic obstacles to navigate, especially in Argentina, and I would recommend buying a Chilean car, preferably in Santiago where there's a critical mass of good used vehicles at reasonable prices.
Buying a Chilean vehicle is relatively straightforward. To do so, you must first obtain a Chilean tax ID (Rol Único Tributario, or RUT) from the Servicio de Impuestos Internos (SII, Chile's Internal Revenue Service), which takes only a few minutes at any SII office). Once you find the appropriate vehicle and agree on a price, there are a few other items to take care of, but it's a fairly simple process that's detailed in Moon Handbooks Chile . After a couple days, you're free to take the car where you like - including over the border into Argentina.
That, in fact, is why I would encourage anyone to purchase a vehicle in Chile rather than Argentina, where the process can be Kafkaesque. As in Chile, non-residents can purchase Argentine vehicles, but there the similarity ends. A few years ago, a U.S. journalist friend of mine who was then living in Buenos Aires (and working for a rival guidebook company) wanted to explore Chile's Carretera Austral. When he arrived at the Argentine border post, in Chubut province, Argentine customs refused him permission to take his Pathfinder into Chile on the grounds that it was a vehículo nacional (Argentine vehicle) that a non-resident foreigner could not take out of the country - even though it was his private property.
This was why, when I purchased my old Peugeot panel truck, we left it in my Argentine father-in-law's name. Ironically and absurdly, a non-resident foreigner can cross the border in a vehicle that is not his or her personal property - with the appropriate power-of-attorney. The upshot, which seems ridiculous, is that you cannot cross the border with your own Argentine vehicle, but you can do so with someone else's.
It gets even more absurd. A few years ago, in southern Argentine Patagonia, I met a young Argentine woman, from the Braun-Menéndez  dynasty of Patagonian wool barons, who had been living for several years in Brazil and had recently married a Brazilian. For their honeymoon, they decided to visit the old family properties in Patagonia and left São Paulo in her car. On arrival at the border, though, Argentine authorities refused them entry on the grounds that an Argentine could not import a foreign car, even temporarily, into the country. Their solution was to return to the Brazilian city of Curitiba, where they obtained a power-of-attorney in the husband's name. On returning to the border, the crossing was routine - except that the same authorities informed them that only he, not she, could drive.
Until recently, that was the silliest story I had heard about Argentine customs, but I recently met, in Puerto Octay, a Swiss family who purchased a 4WD vehicle in Bariloche and attempted to travel to Chile, only to be denied an exit permit at Paso Cardenal Samoré, west of Villa la Angostura. Returning to Bariloche, they contacted the seller and then obtained a power-of-attorney that allowed the seller's brother to drive the car over the border - thus evading Argentine regulations. They then paid the brother's bus ticket back to Bariloche but, when they return to Argentina, they'll have to bring him back in order to cross the border again.