I think it depends on the horse. I have ridden 3 horses in one with 3 entirely different results. All 3 pulled like freight trains which is why I gave it a shot. One acted like I'd put air brakes on him and was a perfect ride in 2 days, one it had literally zero change on, and one improved by about 1/2. Can you borrow one to try?
We used one on a hot little mare that hated a strong bit (would invert & run) but was hard to rate. The kineton was much better than a gag for her, but ultimately, not as good as a hackamore.... I really liked it on her - for a while.
I have one that I pull out on occasion--coming back from time off, riding the first day at a show, etc. Star does better with milder bendy or curved bits (think Sprenger Duo, dynamic RS) and generally as a pretty soft mouth. Except when he decides not to, perhaps following a nifty spin. Or has a Northern Dancer moment of stubbornness. Or both. It basically works exactly as LH describes. There is an after-affect of about one ride, if you are lucky.
A question for those who have used this successfully...do you find that after a period with the kineton, the horse will respond better to your hand when you take it off? Basically I'm wondering if it can be used as a training device only temporarily so the horse learns to respect the bit more, or if the effects are only visible when you use it.