I've paired up at least two people that have "liked" my sites with them linking to them in their blog or on other forums (based on user name matches). I've also bookmarked several sites I've stumbled across, and have shared several as well. In fact, on one of the forums I'm a member at there is a thread for "fantastic stumbles" that lists sites members thought were worthy of sharing via SU...though most are "fun" sites.
its very good traffic, but shitty quality. But it still gets your name out there, people may come back, and it builds backlinks which is why i do it . I receive thousands of visitors per month from stumbleupon maybe just my niche Tyler