Some more detailed stats for SU: Visits: 140,531 Pages/Visit: 4.93 Avg. Time on Site: 00:02:26 % New Visits: 84.98% Bounce Rate: 34.47%
the short answer is yes. I have experienced a great amount of success with StumbleUpon, but you can't abuse it or stumble to many of your stories. I think it is best to pick and choose which pages on your site or blog you think are the most useful or interesting to people, and then stumble it.
Want lots of stumbles? Setup an anime gallery (preferably anime girls or women). Stumble that in the anime section. Last time I tried that, I got about 10,000 uniques in a week. Very little conversions, though, on what I was trying to sell, but lots of bandwidth!
Stumble works but it depends on the site. my site http://copfind.com got a few thousand visitors from stumble.. and a few exchanges and friends stumbling sparked it.
There are about 50 or so stumblers with avs who have liked the front page of the site, but most of the stumbling is on various deeper pages.