But I really wish people would stop getting all up in arms about what these characters are "supposed" to be. They have changed countless times over the decades. Whatever you're thinking of as Lex Luthor is almost certainly not the original. Did the original turn into a superhero and fight alongside Superman in the future, wearing a green and purple fucking robot suit? Fuck no. But that's happened in the comics so much that's it's practically canonical.
My favorite Luthor was from The Dark Knight Striles Again. He was all fucking blood thirsty and brutal and full of murderous evil intent and cruelty. That was fun. But it was nothing like the original Luthor. And the Joker is mostly nowhere near as smart or serious as Heath Ledger's Joker. But Ledger's joker is now "THE" Joker, and it's a damn good thing.
As I said, as Luthor. Eisenberg's villain really doesn't work. But as a new spin on a character that's needed an update for a while... as an experiment... He was fine!
Plus, he's obviously been fucked with by Darkseid. His brain should be a little warbled.
And go see the movie before you start talking about how he's supposed to be a criminal mastermind, because even with the craziness, he does a pretty good job of masterminding the fuck out of the film.