I remember Mark Strong was rumored for Lex, I still believe he would have been a fabulous choice. Strong is older than Affleck and Cavill, but Strong looks like he's in his early 40s; he surprisingly ages gracefully.
I personally don't like the Snyder's Zuckerberg interpretation of Lex; he's beyond a Zuckerberg. LexCorp covers beyond such trivial things as facebook; it covers many divisions of technology, science and innovation. Personally, there are levels of intimidation, depth and range in Lex Luthor's character that I don't think Eisenberg is capable of as an actor. I know I'm going on very little, since maybe Eisenberg was the best actor for the story they wanted to tell, and he may very well work out, but I just think it's an odd choice. Lex Luthor is arguable the best DC villian, and it a little disappointed to me that would go for the polarizing actor for the rest of the DC universe rather than an actor that could best portray the comic version.
But Eisenberg is the guy; and as a fan, all I can do is hope that I am wrong, he plays an amazing Lex that makes the movie better. I don't like the voice, but I have to see him as he talks, as well as the context of his character in the world and story.
Very peculiar casting for a Zack Snyder film, usually his cast choices are just spot on, and I found myself never arguing them(at least until I saw them on film); this one seems a bit oddly cast from the start.