NS have certainly got to pat someone on the back for dreaming up this ticket, as it seems to be their biggest success in recent years. I first wrote about this in October 2013, describing the best strategies to take when booking. It includes which stations in The Netherlands you should pick depending on where you live.
Perhaps this is the ONLY train ticket that hasn't gone up in price (though the terms and conditions allow them to change things at will). You can now purchase a ticket any time until 31st August 2014, for travel up to 31st October 2014, with the exception of King's Day on Saturday 26th April 2014.