They're not really gentle at all. Their design is engineered to stop horses leaning on the bit or just setting themselves and tanking off. It's fluid movement means some horses who like a mobile mouthpiece sometimes go very well in them but they can also often make horses back off the bit *because* it's so fluid.
I've used them for remedial cases (usually the Neck Of Iron types who just run through the bit and ignore you) but would personally always aim to come out of it once that problem is solved. Gelfy wore on for a few months til he learnt he couldn't just charge off and is now in a happy mouth straight bar - proof they can and do work.
I've seen some people saw on a waterford and that is hideous. You're basically dragging chain across the very sensitive bars of the mouth.
So, I guess in summary I'd say they can be very useful, but not a bit I'd want to use for and great length of time (especially when they can cause so many horses to back off or drop behind the bit).