I get some traffic from Stumbleupon and very little conversion. However, I have had some clicks to my site from Stumbleupon. Not adsense clicks. But they have been other clicks. Stumbleupon works pretty good for me. Not the best. But pretty good.
I still see stumble as a good place for social although the fire fox down load for the stumble tool bar has bugs in it""
Stumble readers won't normally click ads, so unless you have CPM based ads it isn't going to help much..
stumble traffic is horrible for some sites, better for others :T takes trial and error i guess. generally entertainment sites do better
SU NoFollows outgoing links so in theory they should be of no value. I do see them in my Google webmaster tools though so it is hard to tell for sure. I think for SU to work well you need something that quickly catches people interest at the very top of the page, otherwise people just click and move on. People On SU are bored, and want something entertaining to distract them.
Google cant do that youre site s domain will get the back link" stumble just getts your blog out in bloggersphere" I think adding the right blpog directories would increase youre readers from stumble even if you do not stumble your content"