Read through this thread. Very informative:
Bit Information (Snaffle and English-Type Bits)
I've seen good results with a Kimberwick on a couple of horses. It's not my preferred bit for the reasons well-explained in the above posts.
My sister-in-law has a couple of rescue-horses. They're TWH from the show circuit. They were ridden in curb bits with chains as the mouthpiece and curb chains. Ewww ewww double-eww. The first thing we did was get rid of those horrid things. I suggested that she put the horses in snaffles to take them back through some basic training and soften them up a bit. Both horses were absolutely confused for some time, then they gradually began understanding the feel of contact and direction. However, the one mare is bred hot - very hot. Every ride with her was a battle for the bit and a workout to the extreme. My sister-in-law and I went back and forth a bit - I feel every horse should be able to respect a snaffle and should be able to be ridden in one if properly trained. She just wanted to be able to ride her horse with some level of control. We met in the middle with the Kimberwick - some leverage for brakes and control, but not so much that she would lose all contact with the mouth. Her horse immediately calmed down, quit fighting all the time, and is easy to control. Horse and rider are very happy. Different horses like/respond to different things, and you never can quite predict what it may be. I like riding a horse in the mildest bit that they can be ridden well in. If your horse is going well in a snaffle and you're getting the performance you want, there is no reason to use a Kimberwick.