He wrote some favorite Superman stories in the '70's. Those years would have been much darker for me without them.
I really loved his "Justices League & Society Meet the Seven Soldiers of Victory" 1972 trilogy in JLA #s 101 & 102 (I missed #100)--just reread those into the ground.
From all accounts, he was a really nice guy.
That JLA three-parter is great no matter how you slice it, but to fully appreciate it, you really need to read the year or two worth of issues that led up to it. Between Denny O'Neill (who's on record as not really liking "super" heroes anyway) and Michael Fleischer with his ham-fisted "social relevance and pretentious "first year lit major" writing style, JLA was one of the most painful reads in comic history. When Len shows up with that story, its an astonishing revelation that maybe the Justice League isn't an inherently unworkable concept after all.
All told, for all the potential you'd expect from an idea like the JLA, good runs have been very much the exception and not the rule.