I got a faulty one, and have not ordered a replacement. However, the fault on mine was a signal amplification fault which made dark and black areas of the screen appear blue. Therefore I could still assess the other functionality to a reasonable degree.
First, this is not as good as using the 480p component connection with a decent cable, but it IS better than using scart (RGB or composite).
The up-scaler works but lacks definition on motion with artefacts showing on, for example, scrolling text. That said, it still beats a scart connection using 576i.
Second, you must configure the Wii to output 60Hz 480p (EDTV) for best results. If you leave it on one of the other 2 modes, the image will be poor.
Mine didn't come with any instructions at all - the small hole on the top is for access to the mode selector. Mine defaulted to 1080p, but using a pen to push this button toggles between that and 720p. Doing so made no difference to the image on-screen, as you would expect; an up-scaled 480p image will look the same at 720p as it will at 1080p. I believe the option is only there to allow compatibility with "HD Ready" TVs and older Plasma screens that are unable to go beyond 720p.
Audio output was fine - I did not test the 3.5 jack, as the audio is carried by the HDMI cable.
I'm giving it 3 stars because this would suit someone without component inputs on their TV, but otherwise not worth it. For the same price or less you can get a component cable and have a much nicer output from your Wii.