Rock tumbling rifle brass?

[wanna-be-sniper]

I recently went to a gun range/shop and i noticed two bags of brass, one before and one after cleaning, and the cleaned brass was like brand new primer pockets and all. I asked the guy behind the counter what they used, he said a rock tumbler with water, dish soap and creme of tarter. Interested if anyone has used this method and pros and cons about it.

thanks