I've used a Pro-9 for several years now - started out with LL and now call high school, Babe Ruth, travel ball, men's league (hell, wherever and whenever they send me!).
I have felt exactly one pitch with this protector, a fastball the catcher clean missed that hit me in the solar plexus and winded me briefly.
That's it.
If you plan on calling higher levels of ball, I would follow Steve's (and others') advice and invest in the Wilson Platinum or like product. My next vest will certainly be of that class.
For 12 and under LL, however, the Pro-9 provides ample protection. Bear in mind, you may start out at LL and then find you have an aptitude for this work, and want to move up.