I don't know about stone prevention, but I LOVE Pu Erh tea. I drink it several times per day as well. It give me great energy. I usually drink it looseleaf with ginger root added. It is great!
I was in China recently, and the Chinese tout it highly... The older Pu Erh sells over there for astronomical amounts of money. They told me it was very healing for the liver and "cleans the blood".
If you do hear about a possible link between Pu Erh and decreased stone formation, I'd love to know. One more reason to buy it!:o)