Octopus user since its introduction
I live in Hong Kong and bought the Octopus card when it was issued in September 1997. At first it was only for public transport, but that alone made it a must-have. Hong Kong Tramways estimated that they save 8% of their operating costs by not having to deal with coins. Buses and the MTR (the tube) also saw benefits.
Because the card was ubiquitous, restaurants and other shops began offering it as a payment gateway. Octopus began offering AAVS: an automatic value-add scheme where the card tops itself up at the touch-processor. I run most daily payments through my Octopus card. It's so popular that at least 12 million cards have been issued--in a city with a population of 7 million (many retired Hong Kongers live across the border).
The systems based on Sony's FeliCa, as noted, and to my knowledge hasn't been compromised--I believe the Oyster technology was at one point. Octopus cards can be customized to provide access to carparks and schools as well.
It's worth noting that the MTRC (operators of the MTR and light-rail system KCR) is privatized and runs at a profit. But they're diversified into property, so they do more than just carry five million passengers daily.
Hong Kong needs this type of technology to handle passenger-loads, but eliminating cash-handling benefits both customers and shop employees. It's an impressive system.