I use the OV-chipkaart, that is an electronic travelcard like the Oyster Card. It doesn't use the optic system, the reader reads the chip on the card. I haven't had problems with it yet, but you must not forget to log in and log out. There are automatic gates at some stations, but they are not in use, so you can walk by with nobody checking you whether you have a ticket. Sometimes I sit in the train and the train guard comes by to check the fares and then I panic because I think I've forgotten to check in, but when the guard checks, I did check in.