I have a Dr. Cook's English style bitless. I've ridden in it about 6
times since I purchased it back in May (weather's been horrific so I
haven't had as much practice as I'd like).

My mare can seize and blow through a bitted bridle on a whoa command.
Stopping her resulted in taking one rein and bending her in a tight
circle until she "listened" and stopped. My instructor says that I am
very light handed, so her reaction wasn't due to me hauling on her
mouth and balancing on my reins, she didn't like the bits (I tried
several types over the course of the past 8 years).

With the bitless, I just sit deep and pull back low and evenly on both
reins and say "whoa" and she stops and I release.

My mare responds very well to this bridle and is more relaxed than she
ever was with a bitted bridle. She's eager and listens.