I have a Stumble Upon button on my blog, but I don't think that people actually use it. I don't do the version where you pay for promotions.
I just signed up and can't really see how this would help promotions. I think just getting your site registered there and on other bookmarking sites helps to raise its visibility, though.
I've been using StumbleUpon for ages; it always gets me a little bit of traffic but I'm not sure that it does anything other than increase my bounce ration. (I don't use the pay option) Most people who come to my blog through it don't stick around -- I keep using it anyway just because it does get people there... it only takes a second to submit a page and if it helps then I figure it's worth it.
Every once in a while I'll have a post that takes off and does really well on SU, but I couldn't tell you why one post gets a lot of hits and others none; there doesn't seem to be any rhyme or reason to it just yet.
I use stumbleupon to put each blog post out there (I don't pay). According to my statistics, each blog post gets on average 2 views referred by stumbleupon. So not a huge traffic generator, no, but it doesn't hurt.
I used StumbleUpon, and it didn't really work for me. :(
I submit each book review to StumbleUpon, but like the others have said, I don't get much traffic and nobody uses the StumbleUpon button on my site.
I get much better results by participating in good, quality blog hops, read-alongs and challenges than by StumbleUpon. Twitter is helpful too, but it takes time to build followers so that you have a decent reach. I haven't really tried Facebook or Google+ yet. I think Facebook could be really helpful, but I'm skeptical about G+.
I've been using it for a while, as well as for my design portfolio website, but I don't see any difference. I don't know if a lot of people use it.
I really don't get it but I stumble all my blogs. One day "Hallmark" was in the title and it was SU'd 5,000 plus times. Perhaps a person searched Hallmark and my blog came up? I don't know. My normal average of SU hits is like 0 to 8.
I used StumbleUpon for some of my posts. It doesn't bring in a lot of traffic for my review posts but I get hundreds of visitors when I stumble my New Releases posts, though it usually takes about a month for me to get results. I have a bit more success with Reddit.