It depends on the guy -- I mean, if you're so veiny that you look like the Hulk, that doesn't seem like it would be too attractive. If you're LEAN, though, you'll be able to see certain veins when you flex, like Van Dam or Hugh Jackman, which appeals to girls long enough to get them looking at your movements. ;-)
If a guy is working out to achieve the veiny look and is proud of it, it is definitely a turn off. I do not want to be around someone showing off their veiny arms. However, if it is due to just trying to get healthy (which is what you are describing) then I would not have a problem with it.