I only know that some specialists recommended it as a bargain at a barcamp a while ago. Your site get's thrown into the stumble stream, so people actually stumble upon your site, they don't have to click on an ad first to get to your site.

Apart from advertising, StumbleUpon also tends to bring a lot of visitors, more than the average digg I think.

I have some anecdotal evidence that may or may not be relevant to technology related customers using stumbleupon.

My wife's craft blog sees spikes sometimes into the 10k unique range (from the usual 1-2k) from stumbleupon, and that's just from someone else recommending (i.e., she's doing nothing to try and get that to happen).