Are third party lenses good on micro four thirds?

Here is what the site says about that.

From Lenses: Third-party lenses: Sigma, Tamron, Samyang and Viltrox make good lenses for many mounts, often for much less than the camera maker's own. Check that the lens supports your body's autofocus features, and that the maker allows third-party lenses on that mount: not every brand does.

From Micro Four Thirds: Micro Four Thirds: One mount shared by OM System, Olympus and Panasonic: every lens fits every body, and lenses are small because the sensor is. Its short flange takes almost any old manual lens through a cheap adapter, at twice the focal length in full-frame terms.

Quoted from this site's pages.