It's possibly worth noting that, even in the comics,
Luthor has been reinvented more than once. I'm old enough to remember when Lex was an evil scientist in a gray prison jumpsuit, before he became a physical threat in green-and-purple battle armor, before he was remade as a ruthless tycoon and future President . . ..
And certainly Hackman, Shea, and Rosenbaum all put their own spin on the character. (Spacey was stuck playing a somewhat more vicious take on Hackman's Luthor.)
The point being that there is NO definitive version of Lex Luthor, so there's always room for a new interpretation. And it seems to me that some of the complaints boil down to "He's not right for my favorite version of Luthor."