I used to only if I was bored - you can find some good stuff via them but now you have to register to use the Firefox plug-in so don't bother anymore.
I personally find most of this kind of stuff via other sources and sites so I kind of browse the opposite way i.e. subject specific to general rather than general stuff to subject specific - if you know what I mean
BTW you need to correct your link the last bit )? is included in the url
I do like stumbling upon pages that I might not have found or thought to find on my own. But it does seem like a waste of time sometimes to sit there and keep hitting the "Stumble!" button till something interesting comes along. After awhile, I found some people's stumbleupon blogs that are interesting so I mostly just go there to see if they've found anything interesting.
There's more work to be done and I haven't added anything new in a LONG time even though I've got about a dozen or so articles that I still need to put the finishing touches on before I will add them to my site.
Stumble Upon is great, as long as you filter your searches to weed out the unnecessary stuff. It lets you stumble onto some of the most interesting and humourous sites on the internet. Of course you also get the pointless and stupid ones, but it's great for when you're bored.