While in Argentina we are not used to sending a Save-the-date card to all the guests, we wanted (read, I came like crazy with the idea and Julio graciously accepted! haha :P) to warn the guests to book our special day. To make it not so formal, we decided to send e-cards, for which we created one in Paperless Post. We chose a topic card and emailed. There are plenty of cards to choose from that are divided by time, some of them are free!
Regarding personal experience, the idea liked among the guests. Anyway, nothing equals the paper so for the formal invitations, which contains the location, date and time, we are dealing with the printer (a funny thing, it is the same printer that made my parents’ invitations 30 years ago!).