In general, I dislike them for all the reasons already mentioned. I particularly dislike being able to look across a schooling area and see, from a hundred yards away, the false head set created by their use. We used to derisively refer to them as "chambon heads."
IF someone rides with tact and feel, and can release the rein pressure as soon as the horse gives, they can be used without negative effect, but of course a rider with that kind of tact and feel can accomplish the same result without draw reins.
I have used them only twice, out of frustration, with OTTBs who were so confirmed in traveling inverted and hollow that no other training method I could find was successful in influencing their way of going. I found that lunging in a surcingle with a draw rein running from high on the surcingle, through a loose ring snaffle and then through the forelegs for gradualling increasing amounts of time helpful. (Both horses mentioned continued traveling inverted and above the bit in donut side reins.) This setup *does* release as soon as the horse gives, and started some muscling on their topline that made it easier for them to go correctly under saddle. I liked the lunging because it allowed me to see how they were using their backs and hindends. I also used this method for about 4 - 6 weeks; once I had made a chink in the hollow/inverted/braced habit we went on from their using conventional methods. I rode in the draw reins them a couple of times and found it was way too easy to abuse them or muddle the "you give, I give" message.
Am I endorsing this method? Not really. As I said, it was out of frustration and after exhausting all the other, non gadget driven, methods I prefer. Somewhere there may have been a rider or trainer so tactful and so consistent that they could have reschooled these horses with a plain snaffle bridle, but I couldn't find them and or learn to be them.