First off--Steven Segal used to teach in L.A. I don't believe his school is there anymore, but he had a Japanese guy teaching (5th Dan I believe) when I visited there, which was about, oh, 16 years ago. I don't think he teaches anymore since, well, a) he's 350 lbs., b) he is a reincarnated guitar playing Tibetan lama.
Jet Li is a real martial artists. Here you go, in his own words:
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Now whether Wushu is effective or not is another question. But the guy has studied it for almost three decades, he obviously can move well and has great technique. You can debate all you want on whether some Pride/UFC fighter could beat him. Fine. But the guy trains hard.
Jackie has never actually claimed to be a serious martial artist so far as I'm aware. But he actually does have a black belt in hapkido.
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As for Wesley Snipes, he is a serious student of the martial arts. He's supposedly done karate, various kung fu styles, and now is doing capoeira and silat styles.
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So there you go. These guys are not wannabes. They might not be the best martial artists in the world, but these guys have logged the hours and done the training. And made a living at it. You've got to give them credit.