Muhammad Saleem, the smartest guy I know when it comes to social media/ voting sites pops up on Mashable to talk how StumbleUpon rocks vs Digg on advertising.
Side note: he’ll never be invited back to post on TechCrunch now he’s appeared there 😉
Seriously though, we ran StumbleUpon advertising through the first 4 odd months of The Inquisitr. I only really stopped it when we really took off, but I can’t recommend it highly enough. Best ad buy you can spend. Contextual down to age group, area and interest. We use to spend $200 at a time, sometimes depending on the campaign it would last days, other times weeks. We always got a boost from each campaign ahead of what we invested. It may only deliver X amount per your spend, but an up vote or even exposure is worth more again in my experience.