I've only used them once.
They're a tool and can be bad or good, depending on the hands utilizing them.
On the horse that I used them on for the most part they remained slack with a loop, however he has issues where he gets EXTREMELY stiff and locked in his jaw due to the way he is ridden by his owner sometimes (she does not ride him in draw reins, thank god). For the most part, he doesn't need them. When he realizes the rider on him has a soft connection he'll work beautifully and they're pretty much just there to remind him to relax in the jaw. A little twitch on the draw reins and he's right as rein (pun intended). In the hour I rode him I used them only once or twice.
I don't condone them in the hands of an inexperienced rider, at all, and I've seen far too much of that. Of the many, many horses I've ever ridden, this is the only time I've found the use of draw reins beneficial, and the next time I ride this horse I probably will just leave them off entirely. It'll be more work on my part, but once he gets going and realizes I won't haul on his face he'll move along quite well.