My son has been using them under the direction of his pitching coach for a year, went from max of 63 mph in November 2005 to 75 mph in August, 2006. There are other things that contributed, like natural strength increases and the like, but I'm convinced the weighted ball routine works. And no injuries last year as an inning gobbling starter.
My concerns with the weighted balls is and always has been that without a very good coach to direct the sessions, bad habits can be developed, like dropping the elbow (trying to throw harder) and changing the release point (higher with the heavier balls, in order for the ball to reach the target). So far, the coach addressed these issues by not throwing to another person 45 or 60 feet away, but throwing at a wall/fence 10 feet away with the coach watching mechanics.