I flew MAD-GRU-MAD in economy on LAN Chile about 18 months ago. I think the plane was a 767. All aspects of the flight - service, comfort and reliability - were just as good as the equivalent class on the main US and European carriers.

So the implication of the thread so far seems to be that the experience is very equipment-specific. Unless you're flying a long distance on one of the 737 horror stories (see above), IMHO, you have no reason to worry.