It is a well-known marketing fact that the average person with a bad product/service experience will tell 10 people, while a good experience will be told to one or none.
Companies know this, and that's why post-sale service exists at all.
If you feel that the product was indeed faulty, then there is nothing wrong with telling people. But, as Phillip said, you should follow up later with what happens. If the company rectifies the situation, kudos for them. Any company occasionally has a dud product, no matter what they make.