Helmet mounted lights "to shine in the eyes of oncoming drivers" (ie to get their attention) isn't a very good idea. For street riding they are probably not worth the cost. Remember, if you want to be treated like a proper road vehicle, behave by the same laws.

Good helmet mounted lights are used for hardcore night mountain biking in conjunction with good bar mounted lights (sometimes both pencil and flood). They serve the purpose of "seeing around corners" as they point in the direction the rider is looking, not in the direction the bar is pointing. This is an advantage when traveling fast of trails with sharp turns or switchbacks. 10w seems to be a practical minimum for such a light.

As to the cars............what can you see when you are driving yours and someone points a high beam or bright light at you**********