We took LAN #533 and #532 nonstops this past January, and J was full in both directions.

On the day that we took #533, the scheduled timing was:
Dp JFK 23:30
Ar SCL 12:05

which we found to be absolutely ideal....there was no traffic to JFK, literally no delay whatsoever in taking off, and the late arrival time facilitated hotel check-in. I'm sure that an 8:50 am arrival in SCL permits many more LAN connections to other parts of Chile/S. America, but in my experience the 8pm departure time from JFK is virtually guaranteed to involve lengthy delays for takeoff.

If you're travelling in J, you're in for a treat. The LAN new J seat is among the best in the air.