I've been improving in the SU field, I do know that I need to get some more SU friends to improve my pull though.
This should be incentive. Since we added Stumble This to all our articles yesterday our traffic and hits showing in Awstats in 24 hours now are as follows. Stumbleupon (Social Bookmark) 37539 240799 WOW
I have read quite a few case studies that reflect this as well. My conversions with StumbleUpon have been minimal at best. I think that if you have a site that is photography or if you have videos on your site, it might keep SU members interested but if you have a blog, it better be filled with a ton of information. And even then, don't expect a big conversion rate (in terms of rss subscribers, ect.)