Well, traditional ben-wa balls are small (3/4 inch), solid and metal. They are suposed to get you off by the way they shift inside you when you go about your business. I have yet to experience that, or hear of anyone outside of the movies who has.

HOWEVER, DuoTone balls are an entirely different story! They are larger- mine are an 1 1/4 inches (you can get them larger or smaller), and they are hollow plastic. Inside each ball is a metal ball bearing that freely moves around and "vibrates" with its movements. The two balls are attatched to each other with a string, and a pull string hangs out of whatever orifice you put them in for easy removal. Now THESE balls get me off! The more you move around, the more intense the vibration. In addition to the vibration, the large size helps, too. You can't stop noticing them once they're in; they won't fall out, and they fill you up. Oh, they only cost about $9 too, and come in several colors.

Personally, I enjoy inserting them just before I exersize (I hate exersizing, and these were just the motivation I needed), and I never fail to have at least one orgasm during the workout. In fact, I work out harder than I normally would because it feels better! I also tend to go out in the evening or to class wearing them, because I get all horned up and ready to go by the time I get home. You should try them! I think they have them at extreme restraints, but don't quote me on that.
-Phantome