Was also sold one by my dentist. Turns out you still have to floss so it was just another thing I paid for at the dentist that I didn't really need. Cool.
try alternately flossing, then using the waterpik, or vice versa. You’ll find that either way, the second modality dislodges food particles left behind by the first. I take that as evidence that both > one > neither in terms of dental health and this of course assumes you then also brush your teeth.