If someone has taken the time to sit down and write a letter of complaint, it's a fair bet that they are very unhappy. If I have reason to complain to a business, I always put it in writing. Sadly, the majority of letters that I have written over the years have simply gone unanswered, which of course makes the complaint even worse.
I wrote to a restaurant once where our meal was a disaster from start to finish. The food was fine, but everything else just fell apart. The restaurant was in a large hotel, so I addressed my letter to the general manager. He rang me as soon as he got the letter to let me know that he would look into it. He was very professional, apologetic and considerate. He asked for all of the details and I was impressed with his approach to handling the complaint.
Within a few days I had received a letter back from him, unreservedly apologizing about the incident and inviting my entire party back to the restaurant for a complimentary meal. We took him up on the offer, went back and had a great time. Now we are regulars and we have been for several years.
This was a very professionally handled complaint. The general manager was smart, and because he handled it so well he not only retained some good customers, he also managed to get us to tell others about the restaurant.
I have had other business owners ring me and abuse me when I have sent them letters. How dare I write to them complaining about my experience and the lack of service? I had one chap screaming at me down the phone. Imagine how much business he has received from me since.
In the same way that if you receive terrible service or have a bad experience you should take the time to write a letter of complaint to the business owner or manager, it is also good practice to write and offer praise when you receive excellent service. Both are great forms of feedback, and it's up to the relevant people to take advantage of them.