It works if used correctly. If you direct the flow down into the gum you can cause the food to get lodged deeper and end up with an infection. Direct the flow away from the gumline.
They are a good adjunctive tool, they do not replace the need for flossing, but rather enhance it. Mostly recoment it for patients with fixed bridge work and poor dexterity.
Dont expect miracles but yes Waterpik is a good oral hygiene tool especially for orthodontic patients, people that have crowns and bridges and find it hard to floss
Add a little bit of Listerine when you use it.