I really liked the 6565 boxcars before I thinned out my roster -- which now tends to be predominately scale-sized rolling stock for O-Gauge and select pieces of tinplate trains for Standard Gauge. Always liked the fact the 6565's had sprung-trucks and were heftier than most traditional rolling stock. Nice idea on Lionel's part, but the series was relatively short-lived... Perhaps 6 or 9 in total. I recall they were only catalog'd two or three times: 3 each time, for ala-carte sale.
I doubt they ever gained much traction as "collectables". Buy 'em 'cause you like 'em. The collectible nature of this stuff tends to change with the wind.
David
