Before the fire gets turned up, lets remember the questions the OP asked
I did parelli for a few months and didn't really enjoy it. I actually found that my mare was more spooky and dominant than she was before. Since I have stopped, she has settled down a lot and is quite calm.
A lot, if not all, of Parelli is common sense stuff, but they just put a different spin on it. While you can get everything used, in every magazine and DVD they're always promoting a different product, that you're going to want and you're going to want to buy... I'm not even going to get into the prices of their tack...
I do encourage you to take bits and pieces from a lot of different trainers and mesh them all together and come up with your own.
But if you do stick with parelli, keep an open mind