I got two of these.  One for my wife and one for me.  I can not tell when she calls me if she is on the phone or using the headset ( maybe that is more a comment on the phone :) ).

The voice interface of the phone has more to do with how well this works.  On my wife's phone she has to train each contact she wants to voice dial.  Not very practical.  On my phone it just figures it out but sometimes I am not certain who it has decided to call.  We both end up dialing through the phone and using the headset to talk and answer calls.  (We both have Verizon WM5 PDA phones.)
I got two of these. One for my wife and one for me. I can not tell when she calls me if she is on the phone or using the headset ( maybe that is more a comment on the phone :) ). The voice interface of the phone has more to do with how well this works. On my wife's phone she has to train each contact she wants to voice dial. Not very practical. On my phone it just figures it out but sometimes I am not certain who it has decided to call. We both end up dialing through the phone and using the headset to talk and answer calls. (We both have Verizon WM5 PDA phones. )