its definitely NOT a scam... I've had mine for over a year... came pretty quickly after I ordered it. Looks identical to the picture on his web site.
Very easy and intuitive to use...
Not sure you'll be able to get enough signal over 50ft of extra serial cable, but you can try.
It definitely will allow you to do a detailed evaluation of your O2 sensors... and will plot the data real time for you.
Let me know how you like it... its a bargain IMHO.
Now, if only we could get something similar to read the 38 pin connector digital signals for the rest of the Mercedes system...