I ride in a rope halter often, my horse also does well in a mechanical hackamore, a sidepull, a cordeo or completely tackless, and we are currently doing a little of bitted training, just for the sake of skills. He was backed in a halter at the age of 4 and has always done great in it, as well as many, many other horses I know who have been started the same way. I love the feeling when you know you don't need any tack and can trust your horse the way he is. :)
And, as others are posting pictures, too...