I use Hawk's, but used to use Porterfields.
I don't like being required to use them, but it is OK - AS LONG AS THEIR CONTINGENCY program works!!
I have had a lot of problems getting pads using their program. Also, their program is not cost effective using a third place certificate vs buying from any of their dealers. Even a second place is about a draw or a slight advantage.
I have had calls and emails into their person since December with
no response. Now I am having our NASA contact help me, but they haven't had any response yet either.