I think it's interesting how a lot of the responses focus on how Luthor is a sympathetic or relateable character, considering that John Byrne's stated that his goal when revamping Luthor was to create a villain who was completely and utterly unsympathetic. Someone whose motivations were not even the smallest extent justifiable.
Just as Superman is someone who does the right thing not because of some childhood trauma or other such defining moment but simply because it's the right thing to do, Byrne wanted a Luthor who was a monster for no other reason than because he chose to be.
I guess he failed?
Well, considering how Byrne portrayed the guy in "The Man of Steel," I suppose that's possible... but can you provide a source?
The reality is that no character who's portrayed as a mustachio-twirling-villain is REMOTELY interesting. Can you name ANY villain who is interesting who is that sort of cardboard cutout?
Now, let's talk about the guy who is still probably the best-recognized "sheer evil" character in modern filmmaking. Hannibal Lector. Lector is undeniably evil by ANY measure, wouldn't you all agree? But that wasn't what made the character so effective, was it? It's not like he's the first murdering cannibal in fiction, after all, is it? And it's not like he's the first "charming" villain, is it?
What make Lector so effective as a source of outright HORROR?
Basically, it's that you're allowed to get into the character's mind during the films. The audience starts to "get" Lector... even (on some strange level) to RELATE to him. That's the real source of horror in that character... not "what he does," but that you, as a member of the audience, don't really MIND him taking off the top of a guys skull and braising a bit of his frontal lobe. Tell me, boys and girls, that you didn't have that reaction...
Where am I going with this? Well, I'm saying that a character is effective ONLY if the audience can see things through their eyes. Luthor is only effective if you, the reader or viewer, can see the world through his eyes.
Doesn't mean you have to AGREE with how he sees it. Just that there has to be a logic to what he's doing that makes sense when seen from his own perspective.
Nobody is saying that they see Luthor as SYMPATHETIC or as "the good guy," are we? No... we're saying that the most interesting version of the character is the one where HE sees himself as "the good guy."
Otherwise, he's just a bland, cardboard character, twirling his figurative mustache and tying Lois Lane to the train tracks just for the sake of "being evil." Which makes for HORRIBLE storytelling, doesn't it?