I've been using it for many years with no problems with fulfillment. I like that there is a record the letter was sent.

The biggest annoyance is the horrible UX for entering recipient's address. I'd like to type the two lines on the envelope. Instead, I need to go through this form adding them as a contact, with fields for e.g. city and state. Most of my letters are one-offs, and it's a bunch of cut-and-paste work. Still beats doing my own mailing, though.