I have been using it and it's great! Paying a little money for some stumbles has brought thousands more free ones. I've noticed that I have gotten a few return visitors from it as well.....
I think their ads are somewhat targeted, not totally random. Good CPC rate, might try Facebook too if it's so low cost.
add me to the list of people who never knew about Stumbleupon having an ad program. heading over there to check it out. As for not being targeted - really depends on your site.
Doesn't seem to be totally untargeted. Anyone have any stats on how the traffic from them behaves? average time? bounce rates?