It was pretty terrible, but it wasn't really him. The portrayal of Lex encapsulates the problem with BvS in general, which is that whoever had control of this thing didn't have faith in the characters or the mythos. Instead of making a cold, calculating Lex who had a philosophy that many could agree with, they opted once again to make the villain the comic relief. The wacky scientist with a personal grudge. The creators didn't have faith that viewers were ready for a complex serious minded villain who also happened to be part of comic book lore. They make Oscar worthy dramas all the time, but they couldn't see the potential in these archetypes?
In this way, it's like WB is fumbling around in the dark.... just not aware of who their audience is or what the value is of their source material. Yes... they had Nolan, who was given a lot of directorial control and made great dramas, but, outside of that, there's just not much indication that WB knows what makes a good comic book movie from a bad one.
Eisenberg could have been a great Lex. He gave a solid performance based on what he had... I guess. The problem was the script. Instead of a maniacal self obsessed xenophobe who cared only for his own power and corporate profits, we got a tool with daddy issues who was twisted by some unknown outside force, basically to the point of insanity. It's discouraging that the creators couldn't see why that would have been a problem.