I've used Datatag/StopTheft transponders to mark my bikes in the past but was wondering whether anyone uses Smartwater to mark a bike. It requires a yearly subscription which makes it look expensive compared to the more usual transponder systems but it also looks like it might be powerful system. Would I be better sticking to a bicycle specific system like Datatag or Immobilise (I think there are others too)?

Usefull for the police to track individual parts. Stolen bikes are ususally broken up and the parts sold of. Your local police may do a
better offer than that offered my smartwater (well they do in Glos).