The Silver Age run is geniunly good. It has a certain heart a lot of pre-Bronze Age comics don't have. The characters have distinct, albeit vague, personalities, and great internal conflicts. Negative Man- a man trapped in a radioactive body! Robotman- a suicidal man trapped in an unfeeling body! They had Marvel style hooks.
The villains were weird, even by Silver Age standards. It had honest to god subplots that ran through at times half a year, giving you an actual sense of time rather than just static adventures. And it has an end. A definite end, and one that is still shocking forty years later.
It all added up to a package that is definitely Silver Age, but quite good regardless. The Showcases are well worth the money on them.