Was about to fill out the survey but realized it didn't ask the right questions to properly portray my opinions on this service.
Personally I think this is a self-defeating service, in that it contradicts itself. The ideal of mobile messaging or email is the speed and ease of delivering said message. However, very idea of sending a personal letter is to some degree, to demonstrate that the sender specifically sacrificed that speed and ease for the sake of the recipient. In other words, it's the thought that counts. Once you deleted that aspect of sending a personalized letter, then you've eliminated the most primary aspect of a personal letter. That's incidentally why I usually don't get cards that have a pre-written message in them (for me it, again, it defeats the point), and instead craft my own message for each letter or card I send.
However, I can see a very few niche applications where this is meant to be used to send messages quickly and conveniently to those who are not wired to receive them electronically; older people, people in first world countries wanting to communicate with people in third world countries. But that's a dead ended path as I don't see that lasting very long as older generations (not to be morbid) die off and the world slowly becomes more wired. Also, in a very ironic way, I could see this being nostalgic or retro for very young generations who really never got to experience a paper-mail world and could find some novelty in it. But again, that would also probably be short lived as well.