The initial spike in traffic from Stumbleupon is only temporary, but there is usually a residual "trail" of traffic that is still coming in days or even weeks afterward if enough people Stumble a page.
Here is one of my pages that got reviewed by 40 people on Stumbleupon and as a result has generated almost 10,000 pageviews over the past few days. Feel free to Stumble it yourself while you're there!
8 Tips on How to Get Out of Bed even when you Don't Want To Stumbleupon review page