The marketing value of "thank you" in business

We recently heard of a SME owner who sent out a New Year’s greeting card to their customers and suppliers with a hand-written thank you message for their support during the year.
It was a small thing, but thoughtful, personal and hugely appreciated judging by all the positive comments it generated on social media, which turned the courtesy into a great piece of marketing.
We also know of a very successful cosmetic surgeon who wrote a hand-written note as a follow-up to the contacts he made and as a result built a successful business which he attributed to his personal attention in writing letters.
As the economic recovery continues increasing numbers of small traders and SME start-ups are appearing and not all of them are offering something unique, so the competition is heating up.
Despite the pace of change and the digital revolution they will need a way to differentiate themselves from the rest.
Hand written and personal thank you messages could be an effective way of doing this and therefore still have a place in the marketing mix.
Have you tried it and with what results?