I just had the Dutch system Klik aan klik uit (kaku), both the old and the new units. Those 2 versions are incompatible and this way I can use both. I used just the new system, but with adding the old units as well I can now make nicer scenes. (turn lights on in a natural 'entering the room' order and turn them off the other way around, mimicking leaving the same room.

I'm just messing about as well and I don't want to use wifi, that's why I'm half looking at zugbee. (And Ikea uses that system, so I guess it'll be available for a while)