Anyone have any experience with the wheel balancing discs such as BalanceMasters or Centramatic? I've read about how they work, with a viscous liquid (usually mercury) inside a sealed channel welded to a disc which you bolt on inside your wheels. Supposedly they constantly adjust to changes in tire speed, road surface, rocks or snow stuck in your tread, etc. It seems like a great idea, but if so why isn't everybody using them from the factory? Anyone have 'em that can give me a heads up BEFORE I shell out the $$.