I only tend to write letters of thanks nowadays. I realised a few years ago that some customer services personnel are just out to fend you off, and quite often do not even appear to have read your (hopefully constructive) letter. What I do now is write (or preferably email) once only on the topic. If they're not interested (which is 99% of the time to my experience) then stuff 'em and go elsewhere with your money. They're getting paid to arse around at the keyboard, you're not.
However, try writing a letter of thanks when you've received good service. I've never received a token or reward of any kind, but the letters you get back are brilliant. It's quite clear that it's rare for them to know that things are going right. Such a shame.