Smallville was another attempt at revitalizing Clark Kent for a younger audience by showing what he was like in the years before he became Superman, but there was one big problem: It was way too successful for its own good. The show was popular enough to run for a full 10 years, meaning that by the end of it, Clark had done pretty much everything you'd expect to see in a decade-long television show about Superman — including working at the Daily Planet, marrying Lois Lane, forming the Justice League, fighting Doomsday, dying, and coming back to life — all before he actually became Superman. The only thing he hadn't done by the series finale was wear his costume.
Since it had to fill a monumental 217 episodes, the show wound up trying to have its cake and eat it too with regards to certain elements of the Superman mythos. The most extreme example of this was a bizarre arc where Jimmy Olsen was killed off and passed down his camera to his young cousin, also named Jimmy Olsen, so that there could still be a Jimmy around to be Superman's pal later. Lex, however, got it as bad as anyone. The original idea was that they'd start the show as friends and grow to become foes by the end of it. Since nobody expected that journey to take 10 years, Lex wound up bouncing back and forth, being manipulated, and eventually having his memory wiped so that he could finally serve as Superman's arch-nemesis — handily erasing all that character development in the process.
That said, Michael Rosenbaum brought an incredible charm and intensity to the role during his tenure on the show, and was frequently the best part of it regardless of where he fell on the good-to-evil alignment chart. The only real problem was that Lex was often overshadowed by his father, Lionel (John Glover), whose delightfully pure, mustache-twirling evil made him a far more entertaining character than Lex, or anyone else on the show. Don't feel too bad for Rosenbaum, though — he also provided the voice of Flash on Justice League Unlimited, and turned out to be just as good at playing the hero as he was at playing the villain.