I have used them.
like Brian,I prefer not to lay a guitar track and over dubbing a vocal,this sytem might produce the best hifi sound and make life easier for the recording engineer, but it [imo]doesnot produce the best music.
on my last two cds, Windy Old Weather and Concertinas and, there were some tracks where it was not physically possible to record the concertina and vocals seperately.
I am convinced that those tracks where I did not record seperately are by far the best tracks.
Brians advice is very good, try and go for the live musicality of musicians playing together as much as possible .Dick Miles