I have had my DD using them for about 2 years now. We have used them in throwing drills and in pitching drills primarily until this past summer. We started using them with a hitting regiment towards the ends of last season and boy did they make a difference.
My DD has always been a decent contact hitter, but was really lacking the power to regularly hit the extra bases. We worked hitting 150 balls a day with the gloves (12oz in each hand) and another 50-150 (depending on the day) with just her regular batting gloves. After two weeks she had developed her upperback enough that she started driving the ball pretty well. Then with a little guidance from Howard Carrier (Thank You again Howard) a small mechanical tweak and viola, she is hitting the fence at least once a game.
I highly reccomend training with these gloves, just be sure to start light while doing any throwing exercises (protect the rotator) and listen to the muscles between the shoulder blades when hitting. When they start to get sore, take the gloves off before you continue your workout. You can't make it all happen at once, it takes time. JMHO
P.S. They have them at Dick's Sporting Goods, but you'll find them cheaper online. I got mine at the link in the post above, they're located in North Ridgeville, OH. and they are very nice to deal with.