I like them. They induce pressure on the poll when the horse raises it's head, and is released as soon as the horse's head drops again - so it is an action brought about by the horse directly. I have always found them to get the horses working in a lovely rhythm on the lunge and working through their backs.
Like anything new, introduce gradually for short periods and then build up.
I do think they have been succeeded by the pessoa though - as this is better for encouraging the horse to work through it's quarters.
I mimick one using draw reins on my horse to encourage it to work through it quarters on the lunge, if I had the spare money id get a pessoa but all I have is pair of draw rein which work quite effectively