I wasn't sure about this one but thought if I could send personal notes a bulk Thank You notes completely branded and on my own domain, I'd give it a try and see the response. I run an online ministry amd 100% of our revenue is gifts offerings and one time donations. At the end of each month I send a thank you email to all who supported that month with an update of highlights of accomplishments.
This month, I downloaded/ uploaded a csv of all our backers for the month and put out a branded thank you card with a personal note and the number of people who wrote back to thank me for the card was astounding. I rarely get a response the monthly email but the card triggered an avalanche. So I'm sold.

The merge fields are quite limited, restricted to name and email address, where I'd like to have some custom mapped fields, for example the donation amount where I could say some thing like "Thank you for your gift of $xxxx". I also find the templates quite dated, so I used my own graphics, but the function and deliverability were quite good.