Not sure how many others feel this way, but I would never go out of my way to contact a business to explain why I don't intend to go there anymore. Especially if it's a customer service issue. If they didn't feel it was important to give good service before, what would make me think that a letter from me is going to change anything?
1. You can't complain to the person who answers the phone; you must go to the owner or at least the manager over it all: they care.