Well if I were going to buy one I'd use NHN as he's sold many to forum members,
Although I saw one recently advertised on eBay which I thought was genuine, I asked about his and this were his reply;
"It comes with the software. It is not original but whoever told you they cause problems is talking rubbish as I've been using it for over a year on my Audi and never had problems, I have also done many coding and scans on peoples cars and they have been happy. At the end of the day, a cable is just a cable and is manufacturered exactly the same, only difference is you can either pay £250+ for one from Ross Tech or you can buy my'n that does exactly the same for just £50. I can show you it all working if you come and collect. "
I asked him that I heard non genuine ones don't work aswell -
Fakes wont get the updates as serial numbers are registered. I do believe fake cables cannot do certain things that genuine cables can......NHN is best to advise on this though as its his 'bread n butter'.
i've used a "cheapo" cable and also tried a mates. they can work fine but my mates struggled with newer models (wouldnt work with B6 A4) and the other worked with every model i tried it on but wouldnt work with windows 7 and seems to work 50% of the time on windows xp.
£250 is a fair whack, especially if you have no immediate need for it but if you can get the cash together it would be well worth it, especially the next time you get a DIS light on and it saves you £40+ at a garage