What's the deal with stumbleupon?

[viewfromthenorth]

Visits to my blog have gone through the roof over the past few days, with nearly all the traffic coming via stumbleupon.com. I had a look at stumbleupon a couple of years back, but didn't use it much, so I was wondering whether anyone else had used it or received much traffic from it. Also, after the initial surge, do traffic levels drop off?