I ran this program for about 2 months. I took precautions like skipping days, running for only 2 hours, shifting the times between stumbles and more. About a week ago I got a note saying my stumble account was banned. Before the ban I had around 700 stumbles given and got back about 450.
I didn't see much of a difference in traffic/pr overall.
If the content your stumbling is crap, then I don't see how this will really benefit you. The purpose is to set off a chain reaction gaining you more stumbles and traffic. But its pointless if nobody likes your content and it looks spammy.
I see this as more a whitehat site tool than blackhat. It can be used to boost your quality whitehat sites.
So should I buy this? I guess he updated the thread pretty recently.