I still have the one I bought when I was doing wireless consultancy out there. It was so useful - just wave a wallet over the MTR pads, and you were in and out of the station. I was commuting from Central to out past Causeway Bay every day, and it was so much easier than throwing coins at a ticket machine. The only complaint I really had was that I was only able to use it on the MTR and main bus routes. Would have been nice to use it in the boats to the outer islands...

Cory - you may have noticed yellow pads on the Tube turnstiles. They're readers being fitted prior to the launch of London's version later this year.

The range is very short, and they appear to be very heavily encrypted. But for a few 10s of HK$ it's a realatively low risk solution...