I've also tried it from the Thunderbolt port on my iMac and this too produces a poor image.
So far I have a couple of theories:
1. Cheap adapter - bad performance.
2. Going from Digital signal to analog - inherently going to be poor pic.
If 1 is true, I don't mind buying the one from Microsoft (even if the price is extortionate!) as long as it does the job properly.
Just wondered if anyone else had used one and what there experience was?