I've used it. The idea was brilliant, but the execution was a little disappointing
The reason is you need an incredibly fast link to the internet to use it properly (both users). I have a cable connection and Sane (the author of the program) had a cable connection too and I was getting more lag than was needed (he was playing fine since he was the server)
When I use this over a LAN, it plays perfectly. That's when you start to get impressed since it's almost guaranteed to have no lag.
Hope that helps
btw - In order to use it, the two of you must have the *SAME ADF* of whichever game as well as the same config in WinUAE (except for Joystick emulation - read the docs on how to set that up)