I would actually prefer not to complain at all, and in cases where it is really necessary to do so, I would most likely complain by phone.
Although, in order to stay within the topic let us compare two given possibilities; complaining in writing and complaining in person.
Complaining in person enables us to interact personally with a real person who represent the company where we purchased a product from. This method of complaining definitely gives certain advantages as opposed to complaining in writing. For instance, one can always ask to speak with a manager if he would find a clerk"s answer not good enough. And this approach also enables us to take the reason of complaining with us and actually demonstrate to the manufacturer or seller what is the actual problem is.
On the other hand, complaining in writing has its own advantages and could serve as a very powerful tool to a sophisticated complainer who knows how to fight for his rights. Firstly, words written on paper could serve as a latter proof in court, if, of course, the dispute would go that far. Also in some cases the company we purchased a product from may not have live representatives available in your region, and therefore, complaining in writing could save you unnecessary long trip. Finally writing a letter with complains could save you a lot of aggravation that you always risk to get by complaining in person.
I personally prefer to complain as less as possible, although if I had to choose from two forms of complaining, I would choose to do it in writing.