Apparently, never....Look at what Joel Schumacher did with the Batman franchise. It looked like the only thing he knew about Batman was the TV series. Then, along came Batman Begins, and that at least gave us a chance to see Batman played straight.
Hollywood has NEVER gotten that it is possible to make a straight drama related to these superheroes. Case in point--the original Superman III (Richard Pryor), and the disatrous Superman IV, (which looked like it was shot as a kid's movie).
Actually, I really enjoy the Gene Hackman portrayal of Lex Luthor. Basically, he's a genius whose mind is so warped that he has turned to villainy. If he was played straight it would probably be a bore. Nah, Lex Luthor is the smartest kid in the 6th grade, all grown up, and smart enough to realize that everything's a joke. That kind of combination of brains, avarice, and criminal vision would probably come out more sardonic than menacing (though he is a tough customer).
The character is peppered with very funny incongruencies. The greatest criminal mind in the world, probably of all time, and he hangs out with idiots. Priceless. The only thing I would change is having him be bald throughout Superman 1 and 2. Spacey seems to borrow heavily from the Hackman vision, AND he is bald, so it looks good to me.
I can't wait for the "Begins" sequel, with The Joker. Should be great. By the way-did you notice all the inmates from Arkham Asylum got away at the end of that movie? That's so they can come back in the sequels as villains. The guy who created Bats sure knew a lot about psychology-Bats and all his adversaries were either schizophrenic, psychotic,or screwed up in some way.