They’ve been most prominent on the Team Sky Pinarellos of Bradley Wiggins and Chris Froome, though other teams’ riders have also fitted them. The two main manufacturers of these weird chainrings are Osymetric or Rotor Q-Rings, however, the technology has been around since the 1980s with Shimano's Biopace chainring.
The idea behind oval chainrings is to eliminate the “dead spot” when your feet are at 12 and 6 and can’t produce much power. The non-round shape should allow you to apply more power through the entire pedal stroke.
Researchers have examined the physiological and performance differences between elliptical and round cranks time and again. And time and again, they’ve concluded that, when it comes to power output, heart rate, perceived exertion, and blood lactate during efforts longer than one kilometer, there’s not much difference between round and elliptical chainrings. However, the manufacturers of these chainrings claim pretty bold claims about performance boosts.
Ultimately, it’s about personal preference. If the Osymetric or Rotor Q-Rings feel more natural to you than a circular chainring, use them. Just don’t expect any amazing insta-transformations from thinking outside the circle.