I've flown Lan to Punta Arenas twice (once when it was a business class service, and once late last year). I have also flown to Puerto Montt at least 4 times. On my first trip, I was in the cockpit of a 767 over Torres del Paine, and it was a memorable special never-to-have-again flight.

If you can get on one of the 319s or 320s, you will be just fine and figure better service than any US carrier on a flight of similar length. Be prepared for strict baggage weights. Avoid the 737s if possible (they are refurbished just fine, but have old and tiny overhead bins, and have no IFE).

I've had moderately good success renting from Econorent, but finding an automatic in Punta Arenas is very difficult.

(I still have 4 open days in next month's trip and am thinking of going back to Chiloe).