The Clipon is really two 300-ml (10-oz) curved flasks, joined together along one side by a spring-loaded hinge. It's made from dishwasher-safe BPA-free plastic and has rubber grips on the inside to help keep it from slipping, and from marring your bike's finish.
It reportedly opens wide enough to fit on a wide variety of frame tubing sizes, plus it can also be used on things like strollers, sailboats or golf carts. Additionally, its Dutch designers point out that different liquids can be kept in the two flasks – water in one and an energy supplement in the other, for example.
Given that it's wider than a traditional bottle, however, we do wonder if it might brush against the inside of riders' legs as they pedal.
If you're interested in finding out for yourself, the Clipon Bottle is currently the subject of a Kickstarter campaign. A pledge of €19 (about US$21) will currently get you one, when and if they reach production.