I've never used one for its intended purpose, but my wife has one. I recently had some food stuck in my gums behind my back tooth, and it got infected. I was able to use the waterpik to remove the food when I wasn't able to get into a dentist for a cleaning right away, and by the time I got the cleaning, between the waterpik and the anti-viral medicine the dentist prescribed, the infection was gone. Having it around saved me quite a bit of pain over a few days.

But I still haven't actually used it for cleaning what it's supposed to. :D