I trained my 8 year old QH mare out of a tie down by using a running martingale. She had an extremely up-side down neck when I bought her from bracing against the tie down. After riding her in the running martingale for some time I finally could leave out the martinagle when tacking her up. I also lunged her in side-reins to redevelope her neck muscles.
Running martingales are really only effective for horses that toss their heads high to avoid the bit. I've only ever had to use it on one horse that I was training for eventing. That particular horse liked to gallop full speed in between jumps on both the x-country course and during stadium jumping. Any attempts I made in slowing him down resulted in him throwing his head up to avoid the bit and galloping like that (dangerous). After a few months of alternating using the running martingale and not using it, he was able to go without it and be controlled.
As long as you have good hands and aren't using it as a permanent fix, but as a training tool, you'll be fine.
I only use a running martingale on my hot TB when we are at a show or xc/hunter pacing. Id rather have it for when he's really hot if he tosses his head in excitement as he sometimes does than not have it. However at home we forego all excess tack - incl no curb rein, no noseband, no martingale, etc. so that we can work on us and the best training possible.
Personally I like a running martingale, it is an aid that, if properly fitted, will only come into play if needed.
I am putting Mr G in one in the spring, when we can get out into the open again. He has a bad habit of throwing his head way in the air and tanking with me, so now we are riding English again on goes the running martingale and see if we can make that safer for both of us.
I have used a running martingale on many horses that I have ridden and trained in the past. If used properly it is a great training tool but never a permanent fix. Also what I like about it more than a standing or tie down is the fact that it only is used if the horse lifts the head past a certain point and still gives them nothing to brace against so helps to teach them to travel properly and round.