I don't mind at all. As long as he's not all 'roided up, I'm fine with it. If a guy is just naturally veiny, I really don't notice. They're kinda cool-looking from a scientific point of view, anyway!
I'm not really a fan of the body-builder look, not when they're extremely into it. And I think it's kind of weird and vain for a guy to spend a whole lot of time lifting weights if he isn't doing it for a sport or something. I like guys who look like they're in shape, but with more of a baseball or soccer player build than a weightlifter.