I use one and I think, being a chinese knockoff adapter the experience of people can vary greatly.
For the most part, the image is the same as component and the Wii U retrocompatibility but slightly darker.
Being displayed in a digital setting instead of analog kills the "color bleed' effect that happens when using analog cables. Even with good quality component cables, the image is not pixel-sharp, especially with vivid tones of red green, blue and white, the color tends to "escape a little" outside it was supposed to.
This is the only advantage of the adapter and it is not caused by itself it is just a consequence of using digital standards, the image is slightly darker but you can achieve this with component by reducing the contrast levels on your TV.
The audio is fine, problem is that it is a little louder than I like.
I use it for two reasons, my TV only has one component port which is currently being used by the PS2 and although it is a cheap knockoff adapter, the response times are quite nice. My TV is ok for analog gaming, but Rhythm Heaven Fever is completely impossible to play on it because it requires the absolute perfect precision and the game for some reason lacks a lag compensation setting, which is completely dumb and I can play this game using the Wii U RC or the wii with wii2hdmi adapter.