It actually does work, but it really depends on how you use it and your overall oral hygiene habits. I tried it for a while and I noticed the difference especially in the spaces between my back teeth where floss always had a hard time getting to properly - my dentist told me the situation had improved, less tartar and gum inflammation.
But those who say it doesn't completely replace floss have a point, meaning the best thing would probably be to do both if you really have the time, but in practice a lot of people don't. If you have to choose between using only the water flosser and using nothing, go ahead with the water flosser; but if you use floss regularly and want to add this as an extra for hard-to-reach spots, it's actually useful. The water waste honestly isn't that bad - if you use it once a day you're using very little water compared to washing your hands.