I wrote a brief, one paragraph email to sjobs@apple.com regarding my 27" iMac. First time I've ever done this (related to the discolored screen, flickering, etc). Within one day I received a phone call from Apple Customer Relations (my email was read by Mr. Jobs and forwarded to the Customer Relations Dept). My iMac is being replaced (get to hold on to the defective one until the new one arrives). The entire process was expedited (the machine was supposed to ship on the 14th of January, it shipped on the 31st of December). There seemed to be a genuine interest regarding the problems I discussed, and I was politely promised that the new machine would not have any of the defects I mentioned in my email. So basically, short and sweet emails work. I'd suggest not resorting to name calling, words of hostility or 'threats' to never buy an Apple product again or anything along those lines. I think most companies are open to constructive feedback and/or complaints about a product one considers to be unsatisfactory. Apple's customer care is truly top notch.
sometimes an emails all it takes!!!
i had problems with sky tv before and emailed their CEO same thing as above, it goes to skys customer excellence dept. my problem was sorted and i had 4 months free tv viewing!
id always recommend sending an email, what have you got to lose?? and for what its worth my email to sky was a full a4 sheet detailing every incident i had with them.