I have one and love it. It removes no end of gunk even after you've cleaned your teeth. I floss and use the water pik before brushing. After all, you sweep the floor before wiping it, don't you - no point in polishing the crumbs.
Fill with warmish water - especially if you have sensitive gums, cold water is very unpleasant and even painful!. It's not recommended or even mentioned in the instructions but I sometimes put a little mouthwash in, too.
Put your thumb or finger on the switch, make sure you know how to switch it on, but don't actually switch it on, put it in your mouth, bend well over sink and then switch it on. You can leave your mouth open a little so that the water runs out, but keep bent over the sink.
I can twizzle the nozzle, so I go along the outside of one side and the inside of the other, which means I don't have to shift positions or do lots of contortions with my wrist, then twizzle the nozzle (without taking the tip of the nozzle out of my mouth) and do the opposite insides and outsides.
It there is any water left in it, I go up and down in between the front teeth as if I was flossing with thread.
It gives a lovely massage.