I have one. I don't use it.
A duplicator is very good at turning square stock round, and ok at replicating a turned shape. It does not, however, give you an identical shape, nor a finish-turned surface. A duplicator gets you close and then you have to complete the turning by hand.
After trying mine out for a while, I decided that it got in the way more than it helped, so I took it off the lathe.
Regards,
Eric