It's one of those things you have to try for yourself, I think. The evidence in my case is very much apparent when I'm in the chair getting poked by my hygienist. The waterflosser is more effective at maintaining my gum health than floss ever was, with a p-value amounting to a floating-point denormal.
I have a feeling that if the truth were to come out, the effectiveness of flossing would turn out to be entirely dependent on tooth spacing or some other individual characteristic. If so, that may be true for the waterjet gadgets as well. But it seems less likely.
