I definitely don't think that it's polite to tell a stranger that they look like a particular celebrity. With a friend or relative that you know well, and you have an idea how they'll take it? Possibly okay, but *only* if the celebrity in question is one that is generally considered to be very attractive to most people (George Clooney, Halle Berry, Denzel Washington, Julia Roberts, etc.), because even if they don't happen to find that actor attractive, chances are they'll recognize that you're saying that you see them as attractive. I could never imagine comparing somebody to Steve Buscemi or anybody who is generally considered to be unattractive and plays roles that reflect that, even if the person really was a dead ringer for him.
It's funny that this comes up, because I had a doctor's appointment the other day, and one of the receptionists at the lab looked *amazingly* like Omar Epps. We're talking truly eerie, especially since he played a doctor on House, M.D. I didn't say anything, though, because who knows if *he* thought he looked like Omar, and what if he thought it was just because they're both black men who he might think have only a superficial resemblance to each other. I really think that unless you're fairly certain of it being taken in a positive way, like any compliment on the appearance, it's best held back.