As hawk20 seems to think it is okay to mention the company I bought mine from, I will take a chance and hope I'm not breaking any forum rules.
The company I dealt with was
http://www.talktomycar.co.uk I bought the U480 although there are some more comprehensive ones available from his site.
The guys name is Mike, and he does post a mobile phone number on his site for people to talk to him (I did and found him very helpful) he also supplies the adapters so you can use the 38 pin connector under the bonnet.
I just looked on his site, and it seems he is away on holiday until the 20th August, whilst I would wait for him, simply because I could ask questions and make certain I get the best one for my needs, I am sure there are other people selling the same code readers.
I used my reader on a W208, but he assured me the reader would work on later MB cars as well, and because I had bought a code reader elsewhere that didn't work on my car, he offered me a money back guarantee so if I had any problems all it would cost me was the postage, I thought that was a pretty good offer (and because he was based in the UK it felt a bit safer!)
Remember though, all the prices on his site are PLUS VAT, I didn't pick up on that when I first looked there.
Hope this helps
PS I have tested my code reader on a Rover 75 a BMW 3 series a Ford focus and a Fiat Brava as well, so far it works with them all, though I did have a little messing around with the wifes 1999 focus to make it work.