Due to the short native flange distance, of the Micro Four Thirds System, the usage of adapted lenses from practically all formats has become widely popular.
Because lenses can be used from old and abandoned camera systems, adapted lenses typically represent good value for the money.
Many lenses easily adaptable to the Micro Four Thirds system with glassless adapters resulting in no induced loss of light or sharpness.
Adapted lenses retain their native focal lengths but have a doubled field of view. An adapted 50mm lens is still a 50mm lens in terms of focal length but has a 100mm field of view due to the Micro Four Thirds System 2x crop factor.
Due to the 2x crop factor of the Micro Four Thirds System however, most adapted glass from the 35mm film era and current DSLR lineups provide effective fields of view varying from normal to extreme telephoto.
Wide angles are generally not practical for adapted use from both an image quality and value point of view.
Using older adapted lenses on micro four thirds sometimes leads to a slight losses in image quality. This is the result of placing high resolution demands on the center crop of decade old 35mm lenses. Therefore, 100% crops from the lenses do not usually represent the same level of pixel-level sharpness as they would on their native formats.
Another slight disadvantage of using adapted lenses can be size. By using a 35mm film lens, one would be using a lens that casts an image circle that is far larger than what is required by Micro Four Thirds Sensors.
Focus is manual even with natively autofocus lenses.
Full metering functionality is maintained however, as are some automated shooting modes (aperture priority).
Overall, the ability to use adapted lenses gives Micro Four Thirds a great advantage in overall versatility and the practice has gained a somewhat cult following. Image samples can be found readily online, and in particular on the MU-43 adapted lenses forum.