I was given one when I had braces. I didn't find it especially useful. The pressure is fairly low so that it doesn't cause pain, and it's noisy. I'd just floss or use the little brushes available in the toothpaste aisle. After I stopped using it, my dad found that it's good for cleaning fingernails, or just very small items that might need a tiny jet of water.
Unfortunately, I broke the end of the hose while trying to remove one of the attachments. Darn cheap plastic parts.