Couldn't agree more, the Stumbleupon guys have done a great job,
however, I will say that their ad panel doesn't always, read never,
shows the current % of +/- votes.

I've contacted them about discounted campaigns and unfortunately they
don't do anything, but that would be a nice plus.

StumbleUpon is indeed a great source of traffic and it is one of the few places online where I will spend any of my ad dollars. They are as transparent as it gets in the ad world. 5 cents and you get a visitor to load whatever page on your site you want… and you can specify what type of people you want to land on that page. The double benefit of this is that you're not just getting another visitor, you're getting a visitor who is active in social media so you could get the benefit of their stumble of your page and begin to build a following of stumblers… then when you cancel your advertising on StumbleUpon you might still get a steady flow of traffic from them as there will now be many stumblers who follow or like your work.

P.S. In terms of watching your stats, I can't agree more. Google Analytics is a little delayed for my taste so I use Mint Stats (http://www.haveamint.com) so I can watch in real time my referrers.

Cheers,
-Darius