I use a pendulum based brake controller with a horse trailer, and thankfully have never been in a panic situation. However, with any sort of tight stopping distance, I'm always very conscious of the brake controller, and often work the manual button which I really wouldn't want to be without.
The ones you definitely don't want are the all electronic timer based ones, which simply apply more breaking pressure the longer the brake pedal has been depressed, as they're completely useless in a panic stop situation.
That's about all I know. I went to the best horse trailer place I could find within about 100 miles of me, and got the only controller they sold. I don't remember the name, but this was a few years ago, and he swore by the pendulum based ones.
The forums over at horstrailerworld.com are where I would go for advice if I needed it, they've always seems like a nice bunch of people whenever I've lurked there.
Matthew