Cool thanks. I never heard of him but thanks again.
If they're talking about the Michael Rosenbaum version, then it's definitely a compliment. If it's the Gene Hackman version, that ain't as good. 
A few days ago, I had a couple YLs' say I look like Lex Luthor, and based upon their ages, I am fairly certain that they have never seen the Gene Hackman version...
OK, I'm very late to the original point of this thread, but it sounds to me like the intent of the comment was negative. Whether or not the most recent incarnation of Lex Luther pulls off the look is irrelevant, especially here. Smallville is on WB...er... CW, in other words it's only seen by about 20 people across the nation. Calling you "Lex" seems like a safe way to insult your look. If you say "skinhead" then it's clearly an insult but "Lex Luther" is a bit more ambiguous. It's a safer way to insult someone.
As for comments on people thinking all sly guys look the same, that doesn't really apply here. Lex has been played by numerous actors and has likely been presented in different ways in the comics and cartoons over the years. In other words, there's really no more look to Lex than there is for Superman. Any buff 30-ish year old white guy with slicked back black hair could be called Superman.