I shoot with direct light; but sometimes put something in between the coin and the light to diffuse it a little, if the coin is too reflective. I have tried a light cube (pictured below) but did not really like the results.
My pictures of coins never look better than the actual coin, in fact, like the old saying, they are better in hand. I shoot in macro with a 100mm lens, so the littlest things can be seen on the coin that in hand are not even noticeable. I have purchased coins from auction houses, mainly German, where the coins sometimes did not hardly look like the picture. Though I sometimes lighten coins if they have a dark patina, otherwise I want the picture to accurately reflect the coin and I think that it is very important that any problems like scratches or flan cracks are clearly shown, especially if I am selling a coin.