I have the flash on a Canon 5D Mk III also used in master-slave operation with another Nissin flash as a slave, it works flawlessly, it being possible to regulate the two halves of the ring separately dimmable.
When used for portraits one must remember that the flash sits exactly in the optical axis and therefore there is often red eyes, which must then be corrected by image processing. Otherwise, however, is also (in addition to the main purpose macro) portrait photography a good application for the ring flash, especially when combined with a second flash as a slave to the illumination of the background. Another application is the reproduction of documents, provided they have no reflective surfaces. I've digitized some old newspaper clippings so z. B..
Five stars for a technically perfect device with a lot of accessories and an unbeatable price-performance ratio.